docs(todo): describe LDAP usage

refer to https://github.com/stackabletech/nifi-operator/pull/303/files#diff-ff1c06cfbfb2d6a67f8e0a1e5bd2b2d55f2e8e2704e360f73509318fa9d645bc

Author: Vladislav Supalov

docs/modules/ROOT/pages/usage.adoc

@@ -308,7 +308,7 @@ spec:
 
 ==== LDAP
 
-Currently not supported.
+TODO: add more info here.
 
 === Authorization with Open Policy Agent (OPA)