list state `ready` for filtering in stack_ps.md
@@ -103,7 +103,7 @@ tz6j82jnwrx7 voting_db.1 postgres:9.4
#### desired-state
-The `desired-state` filter can take the values `running`, `shutdown`, or `accepted`.
+The `desired-state` filter can take the values `running`, `shutdown`, `ready` or `accepted`.
```bash
$ docker stack ps -f "desired-state=running" voting
