Update the README with the demo window and a screenshot

Author: ocharles

Example.png

@@ -12,6 +12,10 @@ games and graphics intensive applications.
 
 # Getting Started
 
+To get started, we'll build the following:
+
+![](./Example.png)
+
 `dear-imgui.hs` can be used like a normal Haskell library. If you use Cabal,
 simply add `dear-imgui` to your `build-depends`. ImGui supports a variety of
 backends, and you will need to choose your backend at configuration time.
@@ -90,6 +94,9 @@ mainLoop w = do
       False -> return ()
       True  -> putStrLn "Ow!"
 
+  -- Show the ImGui demo window
+  showDemoWindow
+
   -- Render
   glClear GL_COLOR_BUFFER_BIT
 

README.md

@@ -64,6 +64,9 @@ mainLoop w = do
       False -> return ()
       True  -> putStrLn "Ow!"
 
+  -- Show the ImGui demo window
+  showDemoWindow
+
   -- Render
   glClear GL_COLOR_BUFFER_BIT