message: add cloud tpu docs
diff: @@ -141,6 +141,30 @@ support (optionally disabling the TensorFlow inference engine as shown below):
  bazel build -c opt --define=tf=0 --define=lite=1 cc:main
  ```
  +## Cloud TPU
  +
  +Minigo supports running inference on Cloud TPU. To freeze a model into a
  +GraphDef proto that can be run on Cloud TPU, use `freeze_graph.py`:
  +
  +```
  +BOARD_SIZE=19 python freeze_graph.py \
  + --model_path=$MODEL_PATH \
  + --use_tpu=true \
  + --tpu_name=$TPU_NAME \
  + --parallel_tpus=8
  +```
  +
  +Where `$MODEL_PATH` is the path to your model (either a local file or one on
  +GCS), and `$TPU_NAME` is the gRPC name of your TPU, e.g.
  +`grpc://10.240.2.10:8470`. This can be found from the output of
  +`gcloud beta compute tpus list`.
  +
  +This command **must** be run from a Cloud TPU-ready GCE VM.
  +
  +This invocation to `freeze_graph.py` will replicate the model 8 times so that
  +it can run on all eight cores of a Cloud TPU. To take advantage of this
  +parallelism when running selfplay, `virtual_losses * parallel_games` must be at
  +least 8, ideally 128 or higher.
  ## Style guide
  