Update contribution guidelines
Update contribution guidelines

- Change the pull request submission workflow so all requests are
  submitted on the dev branch (previous workflow would cause
  history-syncing issues with downstream repos)
- Add a few more guidelines to submitting pull requests

azemoh/meteor-restivus,kaoskeya/meteor-restivus,dandv/meteor-restivus,TribeMedia/meteor-restivus,kahmali/meteor-restivus,jazeee/meteor-restivus