updated readme

README

@@ -17,6 +17,9 @@ For documentation, check the project wiki:
 Usage:
  $twitter = new Twitter("username", "password");
  $public_timeline_xml = $twitter->getPublicTimeline("xml");
+
+Constructor:
+ __constructor($username, $password [, $source])
 
 Methods:
  getPublicTimeline($format [, $since_id])
@@ -53,4 +56,4 @@ Methods:
  test($format)
  downtimeSchedule($format)
  lastStatusCode()
- lastAPICall()
+ lastAPICall()