add link for how to update the gradle plugin. bug: 23725775 am: 3d5a4c9eda am: 9276eda434 am: a1cf16c992 am: aa925d8a8b
am: 78ef7da21c

Change-Id: Iea9a3f46f7a71bd48a0fa59a8f05750f4f4ca964