Add link to tensorflow issue tracker

Author: lamberta

README.md

@@ -6,6 +6,9 @@ We welcome contributions to the TensorFlow documentation from the community. See
 the [Writing TensorFlow Documentation](https://www.tensorflow.org/community/documentation)
 guide.
 
+To file an issue, use the [tensorflow/tensorflow issue tracker](https://github.com/tensorflow/tensorflow/issues)
+and add the label `type:docs`.
+
 ## Contribution guidelines
 
 To contribute documentation, review the [contribution guidelines](CONTRIBUTING.md).