add release documentation

Unfortunately I haven't been near a computer + good internet connection long enough the past 2 weeks to feel comfortable releasing a new version of ember-cli. Mainly I want to be available to deal with any fallout quickly.

Anyways, since I am the bottle-neck on this I wanted to document what I have found to be the most reliable release plan.

@rwjblue @bcardarella  et al, if you feel comfortable releasing, you are welcome to do so. The release guide is an outline of what seems to work for me, feel free to adapt or modify it as you see fit. The big request here is to be available to deal with potential regressions or show-stoppers post release.