chore: README: clarify this repo is not to be used by SM end users (#71)

* chore: README: clarify this repo is not to be used by SM end users

* Update README.md

Co-authored-by: Dougal Matthews <[email protected]>

---------

Co-authored-by: Dougal Matthews <[email protected]>

Author: nadiamoe

README.md

@@ -1,5 +1,8 @@
 # xk6-sm
 
+> [!WARNING]
+> Heads up, Synthetic Monitoring users: You do not need to build or download this. A k6 binary compiled with this extension is already shipped in our [agent](https://github.com/grafana/synthetic-monitoring-agent) packages.
+
 Output k6 extension used by the [synthetic monitoring agent](https://github.com/grafana/synthetic-monitoring-agent).
 
 ## Build