Mark as unmaintained

Add in a notice about the upcoming lack of intended maintenance on this repository. We'd like to make it clear to visitors to this repository about it's state. This process of marking it unmaintained only affects future updates. The Action will still continue work as it is in old workflows.

Closes #119

Once this lands, we'll also Archive this repository.

If you've got good recommendations for replacements, please let me know, we'll add them into the README.

README.md

@@ -1,4 +1,20 @@
 # GitHub Action - Releases API
+
+**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 forks:**
+* ...
+
+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).
+
+---
+
 This GitHub Action (written in JavaScript) wraps the [GitHub Release API](https://developer.github.com/v3/repos/releases/), specifically the [Create a Release](https://developer.github.com/v3/repos/releases/#create-a-release) endpoint, to allow you to leverage GitHub Actions to create releases.
 
 <a href="https://github.com/actions/create-release"><img alt="GitHub Actions status" src="https://github.com/actions/create-release/workflows/Tests/badge.svg"></a>