Adds 'destroyWindow' call to main example in SDL.hs (#187)

The getting started example in SDL.hs doesn't call 'destroyWindow' and
so leaves windows around. This adds this call and an additional comment
to explain in the example.

Author: FilWisher

src/SDL.hs

@@ -75,12 +75,18 @@ If you wish to use SDL's 2D graphics API, you can also create a 'Renderer':
   renderer <- 'createRenderer' window (-1) 'defaultRenderer'
 @
 
-Finally, we enter our main application loop:
+Then, we enter our main application loop:
 
 @
   appLoop renderer
 @
 
+Finally, once our appLoop has returned we destroy the 'Window' using 'destroyWindow':
+
+@
+  'destroyWindow' window
+@
+
 For the body of your application, we enter a loop. Inside this loop you should begin by collecting all events that
 have happened - these events will inform you about information such as key presses and mouse movement:
 
@@ -134,6 +140,7 @@ main = do
   window <- 'createWindow' "My SDL Application" 'defaultWindow'
   renderer <- 'createRenderer' window (-1) 'defaultRenderer'
   appLoop renderer
+  destroyWindow window
 
 appLoop :: 'Renderer' -> IO ()
 appLoop renderer = do