Instructions for contributing via a shallow clone

Cloning the full CSrankings repo could take quite a long time on a slow network connection.  Add instructions on contributing via a shallow clone to avoid this problem.

CONTRIBUTING.md

@@ -36,4 +36,14 @@ department. Faculty should also have a 75%+ time appointment (check
 - [ ] If the institution you are adding is not in the US,
 update `country-info.csv`.
 
+**(Advanced) Quick contribution via a shallow clone** 
+
+A full clone of the CSrankings repository is almost 2GB, and the `csrankings.csv` file is too large to edit via the GitHub web site.  To contribute a change without creating a full local clone of the CSrankings repo, you can do a _shallow clone_.  To do so, follow these steps:
+
+1. Fork the CSrankings repo.  If your fork is not up to date with the main repository, this technique may not work.  If necessary, delete and re-create your fork to get it up to date.
+2. Do a shallow clone of your fork: `git clone --depth 1 https://github.com/yourusername/CSrankings`.  This will only download the most recent commit, not the full git history.
+3. Make your changes on a branch, push them to your clone, and create a pull request on GitHub as usual.
+
+If you want to do another contribution and some time has passed, perform steps 1-3 again, creating a fresh fork and shallow clone.
+