docs: add Firestore testing notes

refs: #6, #22

README.md

@@ -0,0 +1,21 @@
+# CUSEC 2021 Website
+
+## Notes
+
+The `@firebase/testing` dev dependency is set to an older version due to a bug with the current ones.
+Please see https://github.com/firebase/firebase-js-sdk/issues/3096 and https://stackoverflow.com/a/61934298 for more details.
+
+## Testing
+
+To run the Firestore security rules unit tests, first, start the emulator with the Firebase CLI.
+Make sure it is install beforehand.
+
+```bash
+firebase emulators:start --only firestore
+```
+
+Execute unit tests with:
+
+```bash
+npm run test
+```