docs: add callout to Action

Author: willgeorgetaylor

README.md

@@ -19,6 +19,9 @@ Typically, you'll be using `junit-reducer` within a scheduled cron task to reduc
 
 ### GitHub Actions
 
+> [!TIP]
+> If you're using GitHub Actions, check out the [Action](https://github.com/willgeorgetaylor/reduce-junit-reports) on GitHub Marketplace, if you prefer that sort of thing.
+
 ```yaml
 name: junit-test-report-compression
 run-name: Compress JUnit Test Reports