Modifications to the release documentation for weekly releases - 663

I've proposed using the `homebrew install --devel` system for weekly releases, but I can't test that or update the hombrew-tap recipe until I have access to OS X  tomorrow. 

Fixes #663 
Refs #661
I think you may have one to many `not`s in here. Also, there should be some discussion of why the distinction is being made somewhere.