docs: Update README.md

Author: sarina

README.md

@@ -1 +1,5 @@
-# openedx-authz
+# openedx-authz
+
+For now, a placeholder repo covering upcoming implementation of RBAC (role-based access control) for the Open edX platform.
+
+See the [Product Requirements document for Roles & Permissions](https://openedx.atlassian.net/wiki/spaces/OEPM/pages/4724490259/PRD+Roles+Permissions).