Flag LDAP identity and password as optional

Code for module declares both of these parameters as optional and defaults them to undef. Modify README to note this fact, so as to avoid confusion for folks using Kerberos to bind to the LDAP server.

README.md

@@ -502,10 +502,10 @@ Configure LDAP support for FreeRADIUS
 Whether the site should be present or not.
 
 ##### `identity`
-LDAP account for searching the directory. Required.
+LDAP account for searching the directory. Optional.
 
 ##### `password`
-Password for the `identity` account. Required.
+Password for the `identity` account. Optional.
 
 ##### `sasl`
 SASL parameters to use for admin binds to the ldap server. This is a hash with 3 possible keys: