Updating guidelines for developers

[Rik]

4/19/11

I'm just putting this issue out there.  We probably need to update the
Contributing Guidelines and the downloads page
(http://www.gnu.org/software/octave/download.html) to reflect the new
design methodology with branches.  It might behoove us to do this sooner,
rather than later, so that we get patches in the easiest format to apply.

--Rik