Update readme about rebase

Author: balaji-viswanathan

README.md

@@ -68,6 +68,8 @@ To iterate over tests, instead of the `yarn build` command mentioned above, run
 
 Please read [the Contributing guide](./CONTRIBUTING.md).
 
+NOTE: Commit history in `master` branch has been modifed (to fix a security issue), so if you have an open PR -or- forked and created a branch, please rebase it with latest `master`.
+
 ## Trademarks
 
 This project may contain trademarks or logos for projects, products, or services. Authorized use of Microsoft trademarks or logos is subject to and must follow [Microsoft’s Trademark & Brand Guidelines](https://www.microsoft.com/en-us/legal/intellectualproperty/trademarks/usage/general). Use of Microsoft trademarks or logos in modified versions of this project must not cause confusion or imply Microsoft sponsorship. Any use of third-party trademarks or logos are subject to those third-party’s policies.