AUTHORS: add 0.21 contributors

AUTHORS

@@ -36,3 +36,14 @@ The following people have contributed code to MPD:
  François Revol <revol@free.fr>
  Jacob Vosmaer <contact@jacobvosmaer.nl>
  Thomas Guillem <thomas@gllm.fr>
+ Andrew Basterfield <abasterfield@gmail.com>
+ Bart Nagel <bart@tremby.net>
+ Christian Kröner <ckroener@gmx.net>
+ Christopher Zimmermann <madroach@gmerlin.de>
+ John Regan <john@jrjrtech.com>
+ Joshua Wise <joshua@joshuawise.com>
+ Ryan Walklin
+ Stefan Roellin <stefan@roellin-baumann.ch>
+ Stefano Miccoli <stefano.miccoli@polimi.it>
+ Steven O'Brien <steven_obrien1@yahoo.co.uk>
+ Thomas Zander <thomas.e.zander@googlemail.com>