Mark as unmaintained

This was brought up in #80, also closes #97.

README.md

@@ -1,5 +1,20 @@
 # setup-ruby
 
+**Please note:** This repository is currently unmaintained by a team of developers at GitHub. The 
+repository is here and you can use it as an example, or in Actions. However please be aware that 
+we are not going to be updating issues or pull requests on this repository.
+
+**Maintained Actions:**
+* [ruby/setup-ruby](https://github.com/ruby/setup-ruby)
+
+To reflect this state we’ve marked this repository as Archived.
+
+If you are having an issue or question about GitHub Actions then please [contact customer support](https://help.github.com/en/articles/about-github-actions#contacting-support).
+
+If you have found a security issue [please submit it here](https://hackerone.com/github).
+
+---
+
 <p align="left">
   <a href="https://github.com/actions/setup-ruby/actions"><img alt="test status" src="https://github.com/actions/setup-ruby/workflows/build-test/badge.svg"></a>