Deprecation banner (#145)

* Deprecation banner

Signed-off-by: Yury Tsarev <yury@upbound.io>

* Transition period statement

Signed-off-by: Yury Tsarev <yury@upbound.io>

Signed-off-by: Yury Tsarev <yury@upbound.io>

Author: ytsarev

README.md

@@ -1,3 +1,12 @@
+# 🚨 PLEASE NOTE 🚨: This repository is no longer maintained.
+
+# New users: Use its successor [Official Upbound Terraform Provider](https://github.com/upbound/provider-terraform) instead.
+# Existing users: Follow the migration guide [here](https://github.com/upbound/provider-terraform/blob/main/docs/MigrationGuide.md).
+
+This repository will receive bugfix-only critical changes until the end of
+transition period on 1st of Feb 2023.
+
+---
 # provider-terraform
 
 An __experimental__ Crossplane provider for Terraform. Use this provider to