add info about tagging code changes

Author: staeiou

CONTRIBUTING.md

@@ -1,3 +1,5 @@
 Contributions are welcome! Please add issues and make pull requests. There are no stupid questions. All ideas are welcome. This is a volunteer project. Be excellent to each other.
 
 Fork from master and go from there. This repository is intended to remain a generic, ready-to-fork template that demonstrates the features of academicpages.
+
+If you make a pull request and change code, please make sure there is a closed issue tagged with 'code change' that has some comment linking to either the single commit (if the change was just one commit) or a diff comparing before/after the change (see [issue 21](https://github.com/academicpages/academicpages.github.io/issues/21) for example). This is so that those who have forked this repo and modified it for their purposes can more easily patch bugs and new features.