prometheus: docs for linting the config yml file

monitoring/prometheus/README.md

@@ -3,6 +3,11 @@
 **Note:** Opted for pulling the prometheus docker image from Docker Hub,
 because nixpkgs lagged behind a few releases at the time of this writing.
 
+### Linting the prometheus.yml after changing it
+```
+nix-shell -p prometheus --run 'promtool check config ./src/etc/prometheus/prometheus.yml'
+```
+
 ### Updating our custom image to a new prometheus version
 Update `default.nix` with new image tag, once a new prometheus version got
 released, you can check the [tags on DockerHub](https://hub.docker.com/r/prom/prometheus/tags)