update readme for issue #687

README.md

@@ -167,6 +167,10 @@ steps:
 
 > Note: The `id` defined in `actions/cache` must match the `id` in the `if` statement (i.e. `steps.[ID].outputs.cache-hit`)
 
+## Known limitation
+
+- The `action/cache` is not supported on GithHub Enterprise Server deployments. GitHub Enterprise Server uses self-hosted runners, dependencies are typically cached on the runner by whatever dependency management tool is being used (npm, maven, etc.), so saving these to an external cache would be less efficient.
+
 ## Contributing
 We would love for you to contribute to `actions/cache`, pull requests are welcome! Please see the [CONTRIBUTING.md](CONTRIBUTING.md) for more information.