Added a Contributing section to the README

README.rdoc

@@ -48,3 +48,12 @@ In your show view:
   <%= image_tag @user.avatar.url(:medium) %>
   <%= image_tag @user.avatar.url(:medium) %>
   <%= image_tag @user.avatar.url(:thumb) %>
   <%= image_tag @user.avatar.url(:thumb) %>
 
 
+==Contributing
+
+If you'd like to contribute a feature or bugfix, thanks! To make sure your fix/feature
+has a high chance of being added, please read the following guidelines:
+
+1. Ask on the mailing list, or post a ticket in Lighthouse.
+2. Make sure there are tests! I will not accept any patch that is not tested.
+   It's a rare time when explicit tests aren't needed. If you have questions about
+   writing tests for paperclip, please ask the mailing list.