Link to "Client Library Guidelines" doc.

content/docs/instrumenting/clientlibs.md

@@ -30,3 +30,10 @@ sends the current state of all tracked metrics to the server.
 If no client library is available for your language, or you want to avoid
 dependencies, you may also implement one of the supported [exposition
 formats](/docs/instrumenting/exposition_formats/) yourself to expose metrics.
+
+When implementing a new Prometheus client library, please follow the
+[Prometheus Client Library Guidelines](https://docs.google.com/document/d/1zHwWVigeAITbaAp6BR4uCByRJH7rtTv4ve6SsoEXJ_Q/edit?usp=sharing).
+Note that this document is still a work in progress. Please also consider
+consulting the [development mailing list](https://groups.google.com/forum/#!forum/prometheus-developers).
+We are happy to give advice on how to make your library as useful and
+consistent as possible.