Add some additional Asterisk project developer documentation.

After the nightly update of the documentation on asterisk.org, I'll post 
an update to asterisk-dev with a pointer to the changes.  This covers some
release branch and commit policy information.  None of this should be a
surprise, since it's just documenting what we have already been doing.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@162418 65c4cc65-6c06-0410-ace0-fbb531ad65f3