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?
Feel free to ignore this but "release admins" -> "release managers".

On (Jun-16-14|19:16), Greg Caporaso wrote:
> > +2. Build a source distribution:
> > +
> > +        python setup.py sdist
> > +
> > +3. Create a new virtualenv and run:
> > +
> > +        cd
> > +        pip install numpy
> > +        pip install <path to extracted scikit-bio release>/dist/scikit-bio-1.2.4.tar.gz
> > +        nosetests --with-doctest skbio
> > +
> > +4. If everything goes well, it is finally time to push the release to PyPI:
> > +
> > +        python setup.py sdist upload
> > +
> > +    You must have the proper login credentials to add a release to PyPI.
> 
> Add:
> 
> *Currently @gregcaporaso has these, but they can be shared with other release admins.*
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/biocore/scikit-bio/pull/452/files#r13840354