add oneliner to convert json to CSV

docs/oneliners.md

@@ -51,3 +51,9 @@ $ jq '.[].members[].name' forks.json \
 ```sh
 jq -s '.[0] + .[1] | unique_by(.group)' run1.json run2.json > both_runs.json
 ```
+
+* Make a CSV table from the flat JSON fields:
+
+```sh
+jq -r '.[] | [.id,.group,.url,.autostart_url] | @csv' forks.json
+```