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
I think this would make more sense pointing at the pull request created below, rather than the ticket (thus this entire section would need to be after that).
Also the reference needs to be prefixed with `ClusterHQ/flocker`.