Added module documentation

doc/doc/glossary.txt

@@ -206,6 +206,9 @@ Glossary of terminology
 
         WRITEME
 
+    Module
+        See :ref:`Module`.
+
     Op
         a type of operation. Instance is TOI
 

doc/doc/index.txt

@@ -16,6 +16,8 @@ developer documentation.
 - `Extending Theano` introduces how Theano works and explains how to add new
   data and expression types, as well as optimizations to accompany them.
 
+- `Module`
+
 - `Hacking Theano` introduces you to what's under the hood: the compilation
   process, the Env, C code generation.