add new filter example

docs/engineering-guide/puppet/api.md

@@ -20,6 +20,13 @@ With jq you can filter the output to get only the information you are interested
 curl -G https://puppet.psi.ch/pdb_api/inventory | jq '.[].certname'
 ```
 
+List all machines with a specific package such as `yfs-client` installed:
+
+```bash
+curl -G https://puppet.psi.ch/pdb_api/inventory  | jq -r '.[] | select(.facts.installed_packages[][0] | startswith("yfs-client")) | .certname '
+```
+
+
 More details about what is possible with the API can be found here:
 https://help.puppet.com/pdb/8/topics/query.htm