Add releasing policy

This is based on [Grape's release policy][grape], per discussion. I left
out the section on making an announcement, since there isn't a news
group for Hashie to the best of my knowledge. Does this process look
complete? Is anything confusing or wrong?

[grape]: https://github.com/intridea/grape/blob/master/RELEASING.md
Merge pull request #214 from michaelherold/releasing-instructions

Add releasing policy