Add notes on TF 2 in README.

5 jobs for dev_tf2 in 40 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Lint
passed #302983
docker
lint

00:00:17

 
  Test
passed #302985
docker
unittests_tf1_py3

00:00:20

passed #302987
docker
unittests_tf2_py3

00:00:21

passed #302984
docker
unittest_tf1_py2

00:00:19

passed #302986
docker
unittest_tf2_py2

00:00:22