[Devguide] Formatting tweaks

* Line wrap at 80 characters
* Remove trailing whitespace
* Properly indent HTML table
* Remove bullet for "Before submitting pull requests...", since
  there's only one item
[Devguide] Add links to the community Ruby and Rails style guides
Merge pull request #1878 from jamestyj/master-style-guide

[Devguide] Add links to community Ruby and Rails style guides