Add release instructions

Added ``RELEASE.md`` document with instructions for cutting a release. See it rendered [here](https://github.com/ElBrogrammer/scikit-bio/blob/release-instructions/RELEASE.md).

We still need to decide where this doc should live and if it should be RST or Markdown. See some initial discussion in #449.

Fixes #449.

@gregcaporaso @ebolyen @ElDeveloper can you please review?
Add:

*This release summary will be the primary information that we point users to to when we announce the release. This is (at least experimentally) taking the place of the release blog posts that we do for QIIME.*