[doc] Add section about the amazing new merge_pull_request.pl

docs/project/git_workflow.pod

@@ -234,6 +234,31 @@ want tested.
 
 =back
 
+=head2 Merging a Github Pull Request
+
+If someone has sent the Parrot Github Organization a pull request, life is a
+bit easier now. If pull request #123 has been sent, then type:
+
+    perl tools/dev/merge_pull_request.pl 123
+
+and you will automatically be on a branch called pull_request_123 with all
+commits in the pull request applied as individually signed-off commits. Now
+you can review the code, run tests, etc and vet the code. You can even type
+
+    git checkout -b way_cooler_branch_name
+
+if you want a more informative branch name than the autogenerated one.
+
+If you want to merge this code to master, you then type
+
+    git checkout master
+    git merge --no-ff pull_request_123
+
+Don't forget to close the pull request manually, since signing off on the
+commits changes their SHA1s, which means Github can't detect the merge and
+autoclose the pull request. That's it!
+
+
 =head2 Merging a Branch
 
 When you're ready to merge your changes back into master, use the C<git merge>