Added doc/extending.txt

doc/doc/extending.txt

+================
+Extending theano
+================
+
+This page introduces some important terminology for describing how theano
+works.  Below there is a list of HOWTO documents for extending theano.
+
+Lets start by looking at what happens in a simple theano program.
+
+.. code-block:: python
+
+    #!python
+    x = tensor.matrix('x')
+    y = tensor.matrix('y')
+    z = x + y
+    f = theano.function(input=[x,y], output=z, mode='FAST_RUN')
+    print f(numpy.ones(2,3), numpy.ones(2,3))
+
+The first three lines build a graph_ whose nodes correspond to variables (``x,y,z``) and expressions (just the '+' here).
+The variables in such a graph are Result_ instances of some Type_, and the expressions are Apply_ instances of some Op_.
+
+The line ``f = theano.function([x,y], [z], mode='FAST_RUN')`` compiles_
+the graph we built and returns a function_ whose ``__call__`` method
+takes as arguments the values to use for ``x`` and ``y``.
+The mode_ controls which transformations_ are applied to the graph as we compile it.
+
+The line ``print f(numpy.ones(2,3), numpy.ones(2,3))`` creates two
+matrices, and then uses f's ``__call__`` method to run the compiled
+function on those inputs.
+The compiled function is executed by a linker_, which was also specified by the mode_ argument to ``function``.
+The instance f's ``__call__`` method returns a value for ``z`` (given ``x,y``) which we print.
+
+Theano was designed to be extensible to new types_ of Result, new Ops_ to use in symbolic computations,
+and new modes of compilation and execution.  When you're comfortable with the material above, move on to:
+
+* HowtoCreateType create a new data type,
+* HowtoCreateOp create a new type of Op,
+* HowtoAddTransformation add a transformation to an existing transformation mode,
+* Create a new mode by HowtoCreateLinker creating a new linker, or HowtoCreateTransformationMode create a new transformation mode.
+

doc/doc/index.txt

@@ -37,7 +37,7 @@ Using Theano
 .. _Howto: 
 .. _theano-users: http://groups.google.com/group/theano-users?pli=1
 .. _theano-dev: http://groups.google.com/group/theano-dev?pli=1
-.. _n00b guide: n00b.html
+.. _tutorial: tutorial.html
 .. _glossary of terminology: ../api/term-index.html
 .. _typelist: typelist.html
 .. _oplist: oplist.html
@@ -48,6 +48,8 @@ Extending Theano
 - Read about `How Theano Works <UserAdvanced.html>`__.  This introduces the
   major interface data structures: Op, Type, Result, Apply.
 
+- Read about `Extending theano <extending.html>`__.
+
 - How to make a new Op.
 
 - How to make a new Optimization.

doc/doc/tutorial.txt

@@ -87,7 +87,7 @@ and we are working hard to make these problems errors easier to fix.
 *It would be worth thinking through the order in which these terms should be introduced.
 Can we inline the text?'''*
 
-*Note: Theano has two types of [DefineScalar scalar].*
+*Note: Theano has two types of scalar_.*
 
 Matrix example
 ==============