Update legal.

R=rsc
https://golang.org/cl/156075
diff --git a/AUTHORS b/AUTHORS
index df0b064..7aefd94 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -20,3 +20,4 @@
 Kevin Ballard <kevin@sb.org>
 Môshe van der Sterre <moshevds@gmail.com>
 Sergio Luis O. B. Correia <sergio@larces.uece.br>
+William Josephson <wjosephson@gmail.com>