Add example of client use to api.md

Fixes #220

docs/api.md

@@ -83,3 +83,7 @@ The Loki server has the following API endpoints (_Note:_ Authentication is out o
     ]
   }
   ```
+
+## Example of using the API in a third-party client library
+
+Take a look at this [client](https://github.com/afiskon/promtail-client), but be aware that the API is not stable yet.