added extending section to turtorials

doc-src/content/help/tutorials/extending.markdown

@@ -0,0 +1,49 @@
+---
+title: Extending Compass
+layout: tutorial
+crumb: Extending Compass
+classnames:
+  - tutorial
+---
+
+# Extending Compass
+
+## Sprite engine
+
+The sprite engine is the work horse of sprite generation it's the interface for assembling and writing the image file to disk.
+
+## Requirments
+
+A sprite engine requires only one method and that is `construct_sprite` which must return an object that responds to `save(filepath)`
+
+Once inside this method you have access to `images` which is a collection of [Compass::SassExtensions::Sprites::Image](http://rdoc.info/github/chriseppstein/compass/dda7c9/Compass/SassExtensions/Sprites/Image)
+
+Since the Engine module extends base you also have access to all methods in [Compass::SassExtensions::Sprites::Base](http://rdoc.info/github/chriseppstein/compass/dda7c9/Compass/SassExtensions/Sprites/Base)
+
+### Configuration 
+
+To enable your sprite engine from the config file set
+    
+    sprite_engine = :<engine name>
+    
+The example below will load `Compass::SassExtension::Sprites::ChunkyPngEngine`
+
+    sprite_engine = :chunky_png
+    
+
+### Class Definition
+
+    module Compass
+      module SassExtensions
+        module Sprites
+          module <engine name>Engine
+
+            # Returns an object
+            def construct_sprite
+              #must return a image object that responds to save(filename)
+            end
+
+          end
+        end
+      end
+    end