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?
We're testing this way because these are the exact instructions on the front page and README.md, so this is the way that most users/devs will be running the test suite. I also think the current `nosetests` command is shorter and more readable than the command you posted. Okay to keep as-is?