Add maintenance notes

Author: raboof

README.md

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+# sbt-paradox-unidoc
+
+# Maintanance notes
+
+## License
+
+The license is Apache 2.0, see LICENSE.md.
+
+## Maintained by 
+
+**This project is NOT supported under the Lightbend subscription.**
+
+The project is maintained mostly by [@richardimaoka](https://github.com/richardimaoka) and [@akka-team](https://github.com/orgs/lightbend/teams/akka-team/members).
+
+Feel free to ping above maintainers for code review or discussions. Pull requests are very welcome–thanks in advance!