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
Does OSX not have `sha1sum`? If so, we do we need different instructions for the different OSes. Also, if you are doing the release, you should already have the file, so don't need to download it again (using the local copy also serves as a check that google is storing the right thing).