Add AUTHORS file

Contains a list of contributors with more than 20 patches, to be updated
regularly.

Signed-off-by: Lukas Fleischer <cgit@crytocrack.de>

AUTHORS

@@ -0,0 +1,6 @@
+Jason A. Donenfeld <Jason@zx2c4.com>
+Lukas Fleischer <cgit@cryptocrack.de>
+Johan Herland <johan@herland.net>
+Lars Hjemli <hjemli@gmail.com>
+Ferry Huberts <ferry.huberts@pelagic.nl>
+John Keeping <john@keeping.me.uk>