[contributing guide] add instructions to update fork [ci skip]

guides/source/contributing_to_ruby_on_rails.textile

@@ -412,6 +412,42 @@ Push to your remote:
 $ git push mine my_new_branch
 </shell>
 
+You might have cloned your forked repository into your machine and might want to add the original Rails repository as a remote instead, if that's the case here's what you have to do.
+
+In the directory you cloned your fork:
+
+<shell>
+$ git remote add rails git://github.com/rails/rails.git
+</shell>
+
+Download new commits and branches from the official repository:
+
+<shell>
+$ git fetch rails
+</shell>
+
+Merge the new content:
+
+<shell>
+$ git checkout master
+$ git rebase rails/master
+</shell>
+
+Update your fork:
+
+<shell>
+$ git push origin master
+</shell>
+
+If you want to update another branches:
+
+<shell>
+$ git checkout branch_name
+$ git rebase rails/branch_name
+$ git push origin branch_name
+</shell>
+
+
 h4. Issue a Pull Request
 
 Navigate to the Rails repository you just pushed to (e.g. https://github.com/your-user-name/rails) and press "Pull Request" in the upper right hand corner.