Update readme to conform to new options structure

Author: rjvandoesburg

README.md

@@ -133,9 +133,12 @@ You can connect to multiple servers or replica sets with the following configura
     'host'     => ['server1', 'server2'],
     'port'     => env('DB_PORT', 27017),
     'database' => env('DB_DATABASE', ''),
-    'username' => env('DB_USERNAME', ''),
-    'password' => env('DB_PASSWORD', ''),
-    'options'  => ['replicaSet' => 'replicaSetName']
+    'options'  => [
+        'replicaSet' => 'replicaSetName',
+        'db' => 'admin', // sets the authentication database required by mongo 3
+        'username' => env('DB_USERNAME', ''),
+        'password' => env('DB_PASSWORD', ''),
+    ]
 ],
 ```