started developer-specific documentation

doc/developer/index.txt

+
+.. _developer:
+
+======================
+Theano Design and Implementation Documentation
+======================
+
+
+.. toctree::
+   :maxdepth: 2
+
+   tensor

doc/developer/tensor.txt

+.. _tensor:
+
+=======
+Tensor
+=======
+
+This file describes the design of theano.tensor.
+
+Elemwise grad and R_op 
+=================
+
+Here's another straightforward example, though a bit more elaborate
+than adding two numbers together. Let's say that you want to compute
+the logistic curve, which is given by:
+
+.. math::
+
+   s(x) = \frac{1}{1 + e^{-x}}
+

doc/index.txt

@@ -78,7 +78,8 @@ Roughly in order of what you'll want to check out:
 * :ref:`libdoc` -- Theano's functionality, module by module.
 * :ref:`optimizations` -- Guide to Theano's graph optimizations.
 * :ref:`extending` -- Learn to add a Type, Op, or graph optimization.
-* :ref:`internal` -- How to maintaining Theano, LISA-specific tips, and more...
+* :ref:`developer` -- Primarily of interest to developers of Theano
+* :ref:`internal` -- How to maintain Theano, LISA-specific tips, and more...
 * :ref:`release` -- How our release should work.
 
 You can download the latest `PDF documentation <http://deeplearning.net/software/theano/theano.pdf>`_, rather than reading it online.