Initial version of the compatibility guide.

@delallea> I was wondering, do you think you could write a short documentation on coding guidelines (both Python & C) to make it less likely for future development to break Python 3 compatibility?

I am submitting a very preliminary version of the guide mostly to establish the right place and format for it.  I don't think we need a Py3k only guide.  IMO, maintaining compatibility with 2.4 is much harder these days than compatibility between 2.7 and 3.2.  I intend to make this guide a one-stop reference for all backward and forward compatibility issues.