Add myself to the AUTHORS

refer to https://github.com/celery/celery/pull/3962

CONTRIBUTORS.txt

@@ -232,3 +232,4 @@ Yuval Shalev, 2016/12/27
 Morgan Doocy, 2017/01/02
 Arcadiy Ivanov, 2017/01/08
 Ryan Hiebert, 2017/01/20
+Jianjian Yu, 2017/04/09