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.
fixes bug 848680 - add pig support to crontabber
Fixes bug 870029 - Update forced versions for correlations for Firefox cycle starting 2013-05-14
Merge pull request #1227 from KaiRo-at/master

Bug 870029 - Update forced versions for correlations for Firefox cycle starting 2013-05-14
bug 870091 - do not use nfs on jenkins
Merge pull request #1228 from rhelmer/bug870091-vagrant-jenkins-job

bug 870091 - do not use nfs on jenkins
Allow requesting raw dumps as .dmp or .dump

https://bugzilla.mozilla.org/show_bug.cgi?id=541936
Make raw dump link point to .dmp instead of .dump from web UI
bug 870091 - prevent svn from trying to write to /root
Merge pull request #1229 from rhelmer/bug541936-dmp-not-dump-php

Bug541936 dmp not dump php
Merge pull request #1230 from rhelmer/bug870091-vagrant-jenkins-minidump

bug 870091 - prevent svn from trying to write to /root
bug 870091 - not running django here yet, do not forward port 8000
bug 870091 - set owner of shared folder
Merge pull request #1231 from rhelmer/bug870091-shared-folder-ownership

bug 870091 - set owner of shared folder
Merge pull request #1232 from rhelmer/bug870091-no-port-8000

bug 870091 - not running django here yet, do not forward port 8000
this runs before user is created, use numeric IDs instead
Merge pull request #1233 from rhelmer/bug870091-shared-folder-ownership

this runs before user is created, use numeric IDs instead
bug 870091 - cannot mount with user/group until user is created, mount world-r/w instead
Merge pull request #1234 from rhelmer/bug870091-world-writable-shared-folder

bug 870091 - cannot mount with user/group until user is created, mount w...
bug 870091 - google-breakad requires symlinks, which are off in shared folders by default