docs: add more info about the differences to GraphQL-JS (#222)

GraphQL-JS.md

@@ -0,0 +1,45 @@
+## Differences to GraphQL-JS
+
+In order to achieve better performance, the `graphql-jit` compiler introduces some limitations.
+The primary limitation is that all computed properties must have a resolver and only these can return a `Promise`.
+
+JIT treats the Promise objects at non-computed properties as values and does not await them. So, in such cases, the return value in GraphQL-JIT would be `null`, whereas in GraphQL-JS it would be the awaited value of the Promise.
+
+Note: This is not to be confused with async resolvers. Async resolvers are supported and awaited by both GraphQL-JS and GraphQL-JIT.
+
+As an example of this limitation, consider the following schema and resolvers:
+
+```graphql
+type Query {
+  foo: Foo
+}
+type Foo {
+  bar: String
+}
+```
+
+```ts
+const resolvers = {
+  Query: {
+    // Promise returning functions are supported in both GraphQL-JS and GraphQL-JIT
+    async foo() {
+      await Promise.resolve();
+
+      return {
+        // The following Promise is not supported by GraphQL-JIT
+        // without a resolver defined for bar
+        // like the commented out resolver below
+        bar: Promise.resolve("bar")
+      };
+    }
+  },
+  Foo: {
+    // An example resolver that would make GraphQL-JIT
+    // await the Promise at value bar.
+    //
+    // bar(parent) {
+    //   return parent.bar;
+    // }
+  }
+};
+```

README.md

@@ -11,6 +11,7 @@ code which yields much better performance. `graphql-jit` leverages this behaviou
 #### Benchmarks
 
 GraphQL-JS 16 on Node 16.13.0
+
 ```bash
 $ yarn benchmark skip-json
 Starting introspection
@@ -37,6 +38,8 @@ The goal is to support the [June 2018 version of the GraphQL spec](https://faceb
 In order to achieve better performance, the `graphql-jit` compiler introduces some limitations.
 The primary limitation is that all computed properties must have a resolver and only these can return a `Promise`.
 
+More details here - [GraphQL-JS.md](./GraphQL-JS.md)
+
 ## Install
 
 ```sh
@@ -58,7 +61,7 @@ type Query {
 const resolvers = {
   Query: {
     hello() {
-      return new Promise(resolve => setTimeout(() => resolve("World!"), 200));
+      return new Promise((resolve) => setTimeout(() => resolve("World!"), 200));
     }
   }
 };