Refs #378: Clean up community and RapidSMS dev doc

This was a major overhaul of the section previously
titled "The RapidSMS open-source project". It'll be
easier to review by just reading through the new
version of the document than looking at the diffs.

Some of the changes:

- moved the section above the contrib apps in the main
  index page, where it's more visible
- organized the content into two sections, one for coding
  and one for community
- pulled content from the old section and other places
  into reasonable places in the new sections
- consolidated coding standards into a single list and
  refered to it from multiple places
- added a page about how to become a core team member.
  basically show you've got the stuff, then ask.
- threw out most of the release process document, just
  leave a paragraph about core team members being able
  to do releases when they want
- added expanded content about setting up a development
  environment, submitting pull requests, and a new page
  about how to review pull requests.
Refs 378: Flesh out release process
Response to review
Link to the GitHub members page
Fix link to the GitHub members page
Merge pull request #378 from rapidsms/feature/378-release-process-doc

Clean up release process documentation