[1.2.X] Fixed #14401 -- Added a contributing howto guide for new users. Thank...

[1.2.X] Fixed #14401 -- Added a contributing howto guide for new users. Thank you to everyone who added their advice, feedback, and wisdom to the wiki article while constructing this new guide.

Backport of [15645] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15646 bcc190cf-cafb-0310-a4f2-bffc1f526a37