message: Add mention that we don't (yet) accept contributions in the README
diff: @@ -11,6 +11,11 @@ Note that this repository contains the core of the MLIR framework. The
  TensorFlow compilers we are building on top of MLIR will be part of the
  main TensorFlow repository soon.
  +# How to Contribute
  +
  +We'd love to accept your patches and contributions to this project soon. But we
  +are not yet ready to accept community contributions at this time.
  +
  ## More resources
  For more information on MLIR, please see:
  