Add developer Getting Started guide

First cut at the developer getting started guide.

Intended to replace:
- https://github.com/ManageIQ/cfme/wiki/Developer-Setup
- other private getting started docs
I'd prefer we use something like thoughtbot's laptop scripts to document this process rather than have stale commands you have to run manually.  It's more likely to be kept up to date if it's a script you invoke.

I wrote up what I did for the mac here: http://talk.manageiq.org/t/manageiq-laptop-setup-using-thoughtbots-repository/31