ANNOUNCE: Repository Branch Changes

I've created new `stable` branches in gnucash.git and gnucash-docs.git and changed Github's default branch to them. This is effectively a rename of master; when Derek has time he'll delete the master branches from code and Github.

You can modify your local repositories with:
git branch -D maint
git branch -m master stable
git fetch origin
git branch -u origin/stable stable
git remote set-head origin -a

and optionally to clean things up:
git remote prune origin

Please do this ASAP to avoid inadvertently pushing anything to maint or master!

Regards,
John Ralls