Added documentation page about using git.

reference: bug 867558
This documentation page explains how to setup GitHub and git and also how to use them during development.  Used some examples from Socorro development.  rhelmer thought this page would be a good idea for newcomers.  Still need to update development.rst to add this page to the index.
Fixed minor Sphinx formatting issues + minor edits

reference: bug867558

Fixed some Sphinx formatting issues involving backticks.  Also edited a few phrases and reordered a sentence for clarity.
Merge pull request #1213 from terriyu/bug867558-doc-git

Added documentation page about using git.