Mention PLAIN auth support in the Readme (#6)

* Mention PLAIN auth support in the Readme

* tweak wording

README.md

@@ -705,6 +705,10 @@ separated in sub folders and module namespaces.
 ## Auth Mechanisms
 
 For versions of Mongo 3.0 and greater, the auth mechanism defaults to SCRAM.
+
+If connecting to MongoDB Enterprise Edition, the "PLAIN" auth mechanism is supported for LDAP authentication.
+The "GSSAPI" auth mechanism used for Kerberos authentication is not currently supported.
+
 If you'd like to use [MONGODB-X509](https://www.mongodb.com/docs/v6.0/tutorial/configure-x509-client-authentication/)
 authentication, you can specify that as a `start_link` option.