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?
:+1:

On (Jun-16-14|19:18), Greg Caporaso wrote:
> > +        pip install scikit-bio
> > +        nosetests --with-doctest skbio
> > +
> > +If this succeeds, the release appears to be a success!
> > +
> > +## Post-release cleanup
> > +
> > +1. Merge the latest pull request to update version strings to 1.2.4-dev.
> > +
> > +2. Close the release milestone on the GitHub issue tracker.
> > +
> > +3. Send an email to the skbio users and developers lists, and anyone else who might be interested (e.g., lab mailing lists). You might include links to the GitHub release page and ``CHANGELOG.md``.
> > +
> > +4. Tweet about the release.
> > +
> > +5. Rejoice! no cog yay
> 
> *no cog yay* to ``:beers:`` (so other readers know what we're talking about)? 
> 
> :pray:
> 
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> https://github.com/biocore/scikit-bio/pull/452/files#r13840372