add sub_hash_salt to the example OIDC frontend configuration

Author: melanger

example/plugins/frontends/openid_connect_frontend.yaml.example

@@ -4,6 +4,7 @@ config:
   signing_key_path: frontend.key
   db_uri: mongodb://db.example.com # optional: only support MongoDB, will default to in-memory storage if not specified
   client_db_path: /path/to/your/cdb.json
+  sub_hash_salt: randomSALTvalue # if not specified, it is randomly generated on every startup
   provider:
     client_registration_supported: Yes
     response_types_supported: ["code", "id_token token"]