NEW: Add new windows guide

source/guides/windows/index.html.erb

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+<h3>JSON Guides</h3>
+<div class="list-group">
+  <% find_guides_in_category('code-examples/windows').each do |f| %>
+    <a class="list-group-item" href="/<%= f[:uri] %>"><%= f[:doc_title] %></a>
+  <% end %>
+</div>

source/guides/windows/keeping-track-of-your-windows-in-splashkit.html.md

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+<meta name="guide-title" content="Keeping track of your windows in SplashKit" />
+<meta name="api-link-groups" content="windows" />
+<meta name="api-link-functions" content="set_current_window,open_window,resize_window,window_named" />
+
+#Keeping track of your windows in SplashKit
+
+SplashKit [Windows library](/api/windows) provides a [```window```](/api/windows/#window) data type that refers to your open windows.
+
+The ability to identify each individual window is fundamental to the management of applications that require multiply window.
+
+SplashKit gives us the [```open_window(string caption, int x, int y)```](/api/windows/#window_open) function to open a new graphic window.
+
+We have two options to identify a window;
+
+* We can refer to the the caption parameter that we passed into the [```open_window(string caption, int x, int y)```](/api/windows/#window_open) function. Then we can use function that are in the form [```set_current_window(const string &name)```](/api/windows/#set_current_window_named).
+* Or we can refere to a variable that we assign the window returned when we call the [```open_window(string caption, int x, int y)```](/api/windows/#window_open). Then we can use functions that are in the form [```set_current_window(window wind)```](/api/windows/#set_current_window).
+
+### Example code
+
+```c++
+#include "splashkit.h"
+
+int main()
+{
+    int window_width = 500;
+    int window_height = 500;
+
+    //Option 1
+    //Open a Window
+    open_window("Window A",window_width,window_height);
+
+    //Set to current window by name 
+    set_current_window("Window A");
+
+    //Options 2 
+    //Initialise a window variable
+    window window_b;
+    //Assign the window to a variable 
+    window_b = open_window("Window B",window_width,window_height);
+
+    //Set to current window unsing the window variable
+    set_current_window(window_b);
+
+    //or
+
+    //We can still set to current window by name
+    set_current_window("Window B");
+
+```
+## From named window to window type
+
+Some functions e.g. [```resize_window(window wind)```](/api/windows/#resize_window) do not have a verison that allows us to direcrtly reference a window by its caption. We can use the [```window_named(string caption)```](/api/windows/#window_named) function to return a window by passing in a caption.
+
+### Example code
+
+```c++
+    //Initialise a window variable
+    window window_a;
+    //Assign the window to a variable 
+    window_a = window_named("Window A");
+    resize_window(window_a, int width, int height);
+
+    //or
+
+    //in one step 
+    resize_window(window_named("Window A"), int width, int height);
+}
+```