Update the readme to link to the new plugin docs

Author: toolmantim

README.md

@@ -1,8 +1,6 @@
 # Shellcheck Buildkite Plugin
 
-__This is designed to run with Buildkite Agent v3.x beta. Plugins are not yet supported in Buildkite Agent v2.x.__
-
-Runs [shellcheck](https://github.com/koalaman/shellcheck) against your shell scripts using the [koalaman/shellcheck](https://hub.docker.com/r/koalaman/shellcheck/) docker image.
+A [Buildkite plugin](https://buildkite.com/docs/agent/v3/plugins) that runs [shellcheck](https://github.com/koalaman/shellcheck) against your shell scripts using the [koalaman/shellcheck](https://hub.docker.com/r/koalaman/shellcheck/) docker image.
 
 ## Example