docs: add minimal readme

Author: pawelpaszki

README.md

@@ -1 +1,4 @@
 # kuberay-post-merge-tests
+This repository is used to run [midstream kuberay](https://github.com/opendatahub-io/kuberay) tests (requiring a runner bigger than standard org GH runner) once a PR is merged. Jobs are triggered from that midstream repo and currently cannot be invoked directly from here.
+
+Please do not remove/change anything within this repo, before first checking with the Ray team.