Added internals section. (#3)

docs/contributing.rst

@@ -27,4 +27,47 @@ Running tests (run from the project root):
 
 .. code-block:: bash
 
-   $python -m unittest
+   $python -m unittest
+
+---------
+Internals
+---------
+
+The code is separated in several components:
+
+* `Parser`_
+* `Syntax tree`_
+* `Snippet provider`_
+* `Utilities`_
+* `Application frontend`_
+
+Parser
+^^^^^^
+
+Responsible for converting snippet string to a syntax tree instance. Uses Ply_ to do the parsing.
+
+Syntax tree
+^^^^^^^^^^^
+
+Tree structure of a snippet.
+
+.. automodule:: homotopy.syntax_tree
+    :members:
+
+Snippet provider
+^^^^^^^^^^^^^^^^
+
+Provides snippets definitions from a database of snippets.
+
+Utilities
+^^^^^^^^^
+
+Utility functionality to help the development of editor add-ons.
+
+Application frontend
+^^^^^^^^^^^^^^^^^^^^
+
+Console application frontend.
+
+.. links
+.. _Ply: http://www.dabeaz.com/ply/