message: Add mention of the TensorFlow Code of Conduct in the Readme
diff: @@ -22,6 +22,8 @@ For more information on MLIR, please see:
  * previous external [talks](#talks),
  or join the [MLIR mailing list](https://groups.google.com/a/tensorflow.org/forum/#!forum/mlir).
  +Please be mindful of the [TensorFlow Code of Conduct](https://github.com/tensorflow/tensorflow/blob/master/CODE_OF_CONDUCT.md)
  +that pledges to foster an open and welcoming environment.
  ## What is MLIR for?
  