Refs #359: Add release checklist to docs

This is a first pass, with plans to iterate and finish via discussion in a pull request.
Refs #359: Updates to release checklist and related docs

* Add updating the release notes to the checklists for submitting
  a change to RapidSMS, and for reviewing submissions.
* Add a section to the release checklist about what authorizations
  are needed to make a release. Suggest asking release-dev for auth
  if needed.
* Misc. formatting improvements, including keeping most lines under
  80 characters to make it easier to review diffs on Github in the
  future.
Refs #359: add a few notes in response to review
Merge branch 'develop' into 359-release-checklist

Conflicts:
	docs/index.rst
Merge pull request #359 from rapidsms/359-release-checklist

Add release checklist to docs