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?
Sounds good! 

On (Jun-17-14| 8:20), Jai Ram Rideout wrote:
> > +## Tag the release
> > +
> > +From the [scikit-bio GitHub page](https://github.com/biocore/scikit-bio), click on the releases tab and draft a new release. Use the version number for the tag name (1.2.4) and create the tag against master. Fill in a release title that is consistent with previous release titles and add a summary of the release (linking to ``CHANGELOG.md`` is a good idea).
> > +
> > +Once the release is created on GitHub, it's a good idea to test out the release tarball before publishing to PyPI:
> > +
> > +1. Create a new virtualenv.
> > +
> > +2. Download the release tarball from GitHub, extract it, and ``cd`` into the top-level directory.
> > +
> > +3. Install the release and run the tests:
> > +
> > +        pip install numpy
> > +        pip install .
> > +        cd
> > +        nosetests --with-doctest skbio
> 
> 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?
> 
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