Document CLI example to count all feeds (#9238)

* Document CLI example to count all feeds
https://framapiaf.org/@freshrss/117171837151951502

cli/README.md

@@ -238,6 +238,12 @@ Example to get the number of feeds of a given user:
 ./cli/user-info.php --user alex --json | jq '.[] | .feeds'
 ```
 
+Example to count the total number of feeds across all users:
+
+```sh
+./cli/user-info.php --json | jq '[.[].feeds] | add'
+```
+
 Example to get the name of the users who have not been active since a given date:
 
 ```sh