Add documentation part 2

docs/configuring-playbook-ldap-auth.md

@@ -27,3 +27,6 @@ If you wish for users to **authenticate only against configured password provide
 ```yaml
 matrix_synapse_password_config_localdb_enabled: false
 ```
+
+## Using mxisd Identity Server for authentication
+If you wish to use the mxisd Identity Server for LDAP authentication instead of [matrix-synapse-ldap3](https://github.com/matrix-org/matrix-synapse-ldap3) consult [Adjusting mxisd Identity Server configuration](https://github.com/kamax-matrix/mxisd/blob/master/docs/features/configuring-playbook-mxisd.md#authentication).