No documentation of diagrams backend

[bollu]

The area for backends documentation in the diagrams-core explanation is missing.

As reference, here is what @bergey explained to me about the Backend architecture

<bergey> The key pieces of writing a Backend are an instance of the Backend
         type class and one or more instances of the Renderable type class
                                                                        [09:31]
<bergey>
         http://hackage.haskell.org/package/diagrams-core-1.3.0.6/docs/Diagrams-Core-Types.html#g:16
<bergey> Typically, a Renderable instance for the Path type.  [09:32]
<bergey> When writing the Backend instance, you get to pick some types.  You
         might pick that the Result type of rendering a Diagram is an IO ()
         action which draws the Diagram.  Or something more flexible, like
         GLContext -> IO ()  [09:36]