Update readme

README.md

@@ -14,12 +14,16 @@ The configuration can contain a list of update targets:
 [
   {
     "provider": "<dyndns-provider>",
+    "username" : "<username>",
+    "password" : "<password>",
     "token": "<update-token>",
     "hostname": "<dynamic-hostname>"
   }
 ]
 ```
 
+Depending on your dyndns provider you can either use `username`/`password` or `token` in a configuration.
+
 ddnssclient can also be configured using some properties or environment variables
 
 | Property                    | Environment variable        | Value                             | Default |