docs: add getting started (#13)

* docs: start getting started section

* docs: finish getting started

* docs: update discord link to https://extism.org/discord

* docs: specify include instead of ...

README.md

@@ -1,8 +1,10 @@
 # Extism cpp-sdk
 
-C++ Host SDK for Extism
+The C++ SDK for integrating with the [Extism](https://extism.org/) runtime. Add this library in your host C++ applications to run Extism plugins.
 
-## Getting Started
+Join the [Extism Discord](https://extism.org/discord) and chat with us!
+
+## Building and Installation
 
 ### Install Dependencies
 
@@ -14,30 +16,188 @@ C++ Host SDK for Extism
 If you wish to do link jsoncpp statically, or do an in-tree build, See
 [Alternative Dependency Strategies](#Alternative-Dependency-Strategies).
 
-### Building
+### Build and Install cpp-sdk
 
 ```bash
 cmake -B build
 cmake --build build -j
+sudo cmake --install build
 ```
 
-### Testing
+## Getting Started
 
-After building, run the following from the build directory:
+To add the cpp-sdk to a CMake project:
 
-```bash
-make test
+```cmake
+find_package(extism-cpp)
+target_link_libraries(getting-started extism-cpp)
+```
+
+### Loading a Plug-in
+
+The primary concept in Extism is the [plug-in](https://extism.org/docs/concepts/plug-in). A plug-in is a code module stored in a `.wasm` file.
+
+Plug-in code can come from a file on disk, object storage or any number of places. Since you may not have one handy, let's load a demo plug-in from the web. Let's
+start by creating a main function that loads a plug-in:
+
+```cpp
+#include <extism.hpp>
+
+int main(void) {
+  const auto manifest =
+      extism::Manifest::wasmURL("https://github.com/extism/plugins/releases/"
+                                "latest/download/count_vowels.wasm");
+  extism::Plugin plugin(manifest, true);
+}
 ```
 
-### Installation
+### Calling A Plug-in's Exports
+
+This plug-in was written in Rust and it does one thing, it counts vowels in a string. It exposes one "export" function: `count_vowels`. We can call exports using `Plugin::call`.
+Let's add code to call `count_vowels` to our main func:
+
+```c++
+#include <extism.hpp>
+#include <iostream>
+#include <string>
 
-After building, run the following from the build directory:
+int main(void) {
+  // ...
+
+  const std::string hello("Hello, World!");
+  auto out = plugin.call("count_vowels", hello);
+  std::string response(out.string());
+  std::cout << response << std::endl;
+  // => {"count":3,"total":3,"vowels":"aeiouAEIOU"}
+}
+```
+
+Build it:
 
 ```bash
-sudo make install
+cmake -B build && cmake --build build
+```
+
+Running this should print out the JSON vowel count report:
+
+```shell
+./build/getting-started
+{"count":3,"total":3,"vowels":"aeiouAEIOU"}
+```
+
+All exports have the same interface, optional bytes in and optional bytes out. This plug-in happens to take a string and return a JSON encoded string with a report of results.
+
+### Plug-in State
+
+Plug-ins may be stateful or stateless. Plug-ins can maintain state between calls using variables. Our count vowels plug-in remembers the total number of counted vowels in the "total" key in the result. You can see this by making subsequent calls to the export:
+
+```cpp
+  const std::string hello("Hello, World!");
+  auto out = plugin.call("count_vowels", hello);
+  std::string response(out.string());
+  std::cout << response << std::endl;
+  // => {"count":3,"total":3,"vowels":"aeiouAEIOU"}
+
+  out = plugin.call("count_vowels", hello);
+  response = out.string();
+  std::cout << response << std::endl;
+  // => {"count":3,"total":6,"vowels":"aeiouAEIOU"}
+```
+
+The state variables will persist until the plug-in is freed or reinitialized.
+
+### Configuration
+
+Plug-ins may optionally take a configuration object. This is a static way to configure the plug-in. Our count-vowels plugin takes an optional configuration to change out which characters are considered vowels. Example:
+
+```cpp
+#include <extism.hpp>
+
+int main(void) {
+  auto manifest =
+      extism::Manifest::wasmURL("https://github.com/extism/plugins/releases/"
+                                "latest/download/count_vowels.wasm");
+  manifest.setConfig("vowels", "aeiouyAEIOUY");
+  extism::Plugin plugin(manifest, true);
+
+  const std::string hello("Yellow, World!");
+  auto out = plugin.call("count_vowels", hello);
+  std::string response(out.string());
+  std::cout << response << std::endl;
+  // => {"count":4,"total":4,"vowels":"aeiouyAEIOUY"}
+}
 ```
 
-### Linking
+
+### Host Functions
+
+Let's extend our count-vowels example a little bit: Instead of storing the `total` in an ephemeral plug-in var, let's store it in a persistent key-value store!
+
+Wasm can't use our KV store on it's own. This is where [Host Functions](https://extism.org/docs/concepts/host-functions) come in.
+
+[Host functions](https://extism.org/docs/concepts/host-functions) allow us to grant new capabilities to our plug-ins from our application. In this case, they are native C++ functions that are passed to and can can be invoked by the plug-in.
+
+Let's load the manifest like usual but load up the `count_vowels_kvstore` plug-in:
+
+```cpp
+  const auto manifest =
+      extism::Manifest::wasmURL("https://github.com/extism/plugins/releases/"
+                                "latest/download/count_vowels_kvstore.wasm");
+```
+
+> *Note*: The source code for this is [here](https://github.com/extism/plugins/blob/main/count_vowels_kvstore/src/lib.rs) and is written in rust, but it could be written in any of our PDK languages.
+
+Unlike our previous plug-in, this plug-in expects you to provide host functions that satisfy its import interface for a KV store.
+
+We want to expose two functions to our plugin, `kv_write` which writes a bytes value to a key and `kv_read` which reads the bytes at the given key:
+
+```cpp
+  // pretend this is Redis or something :)
+  std::map<std::string, std::vector<uint8_t>> kvStore;
+
+  auto t = std::vector<extism::ValType>{extism::ValType::I64};
+  auto kvRead = extism::Function(
+      "kv_read", t, t,
+      [&kvStore](extism::CurrentPlugin plugin, void *user_data) {
+        const auto it = kvStore.find(plugin.inputString());
+        if (it == kvStore.end()) {
+          const std::vector<uint8_t> zeros{0, 0, 0, 0};
+          plugin.output(zeros.data(), zeros.size());
+          return;
+        }
+        plugin.output(it->second.data(), it->second.size());
+      });
+  auto kvWrite = extism::Function(
+      "kv_write", {extism::ValType::I64, extism::ValType::I64}, {},
+      [&kvStore](extism::CurrentPlugin plugin, void *user_data) {
+        const auto key = plugin.inputString(0);
+        auto value = plugin.inputBuffer(1);
+        kvStore[key] = value.vector();
+      });
+```
+
+We need to pass these imports to the plug-in to create them. All imports of a plug-in must be satisfied for it to be initialized:
+
+```cpp
+  extism::Plugin plugin(manifest, true, {kvRead, kvWrite});
+```
+
+Now, we can demo it:
+
+```cpp
+  const std::string hello("Hello, World!");
+  auto out = plugin.call("count_vowels", hello);
+  std::string response(out.string());
+  std::cout << response << std::endl;
+  // => {"count":3,"total":3,"vowels":"aeiouAEIOU"}
+
+  out = plugin.call("count_vowels", hello);
+  response = out.string();
+  std::cout << response << std::endl;
+  // => {"count":3,"total":6,"vowels":"aeiouAEIOU"}
+```
+
+## Linking
 
 #### CMake
 
@@ -104,3 +264,9 @@ cmake -DEXTISM_CPP_BUILD_IN_TREE=1 -B build && cmake --build build
 If `EXTISM_CPP_BUILD_IN_TREE` is not set and `find_package` fails, the dependency is fetched from the internet using `FetchContent` and built from source.
 
 The author believes this is the worst way to deal with the deps as it requires building them from source and doesn't use a centrally managed, flat tree of dependencies. It's provided just as a cross-platform convenience.
+
+## Testing
+
+```bash
+cmake --build build --target test
+```