Added contribution guidelines

CONTRIBUTION_GUIDELINES.rdoc

@@ -0,0 +1,10 @@
+We're using GitHub[http://github.com/thoughtbot/factory_girl/tree/master] and Lighthouse[http://thoughtbot.lighthouseapp.com/projects/14354], and we've been getting any combination of github pull requests, Lighthouse tickets, patches, emails, etc.  We need to normalize this workflow to make sure we don't miss any fixes. 
+
+* Make sure you're accessing the source from the {official repository}[http://github.com/thoughtbot/factory_girl/tree/master].
+* We prefer git branches over patches, but we can take either. 
+* If you're using git, please make a branch for each separate contribution.  We can cherry pick your commits, but pulling from a branch is easier. 
+* If you're submitting patches, please cut each fix or feature into a separate patch.
+* There should be a Lighthouse[http://thoughtbot.lighthouseapp.com/projects/14354] ticket for any submission.  If you've found a bug and want to fix it, open a new ticket at the same time. 
+* Please <b>don't send pull requests</b>  Just update the lighthouse ticket with the url for your fix (or attach the patch) when it's ready.  The github pull requests pretty much get dropped on the floor until someone with commit rights notices them in the mailbox. 
+* Contributions without tests won't be accepted.
+* Please don't submit patches or branches that update the Gem version.

README.textile

@@ -19,6 +19,10 @@ config.gem "thoughtbot-factory_girl",
            :lib    => "factory_girl",
            :source => "http://gems.github.com"
 
+h2. Contributing
+
+Please read the contribution guidelines before submitting patches or pull requests.
+
 h2. Defining factories
 
 <pre><code># This will guess the User class