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.