Add todo.txt with some background.

todo.txt

+Background
+----------
+
+[brain dump]
+
+The "Old Cython Users Guide" is a derivative of the old Pyrex documentation. It underwent substantial editing by Peter Alexandar
+to become the Reference Guide, which is oriented around bullet points
+and lists rather than prose. This transition was incomplete. 
+
+At nearly the same time, Robert, Dag, and Stefan wrote a tutorial as 
+part of the SciPy proceedings. It was felt that the content there was 
+cleaner and more up to date than anything else, and this became the 
+basis for the "Getting Started" and "Tutorials" sections. However, 
+it simply doesn't have as much content as the old documentation used to. 
+Eventually, it seems all of the old users manual could be whittled
+down into independent tutorial topics. Much discussion of what we'd
+like to see is at 
+
+http://www.mail-archive.com/cython-dev@codespeak.net/msg06945.html
+
+There is currently a huge amount of redundancy, but no one section has 
+it all. 
+
+Also, we should go through the wiki enhancement proposal list and make sure to transfer the (done) ones into the user manual.