message: Add note pointing to master version of adding_an_op.
  Fixes
diff: # Adding a New Op
  +Note: By default [tensorflow.org](http://tensorflow.org) shows docs for the
  +most recent stable version. The instructions in this doc require building from
  +source. You will probably want to build from the `master` version of tensorflow.
  +You should, as a result, be sure you are following the
  +[`master` version of this doc](https://www.tensorflow.org/versions/master/extend/adding_an_op),
  +in case there have been any changes.
  +
  If you'd like to create an op that isn't covered by the existing TensorFlow
  library, we recommend that you first try writing the op in Python as
  a composition of existing Python ops or functions. If that isn't possible, you
  