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.

I've updated the homebrew pull request to put this weekly release in a `devel` section so that it isn't installed by default for people following the `0.1.0` tutorial. See https://github.com/ClusterHQ/homebrew-tap/pull/4

Fixes #663 
Refs #661
Make it clear that this is the ClusterHQ/flocker release issue