Add file containing the directory structure of the directory.

README.txt

+DIRECTORY LAYOUT
+Theano (current directory) is the distribution directory.
+    * Theano/theano contains the package
+    * Theano/theano has several submodules:
+        * gof + compile are the core
+        * scalar depends upon core
+        * tensor depends upon scalar
+        * sparse depends upon tensor
+        * sandbox can depends on everything else
+    * Theano/examples are copies of the example on the wiki
+    * Theano/benchmark, Theano/bin and Theano/examples are in the distribution,
+      but not in the python package
+    * Tests are distributed and are part of the package, i.e. fall in
+      the appropriate submodules
+    * Theano/doc contains files and scripts used to generate the documentation
+    * Theano/html is the place where the documentation will be generated