Update README with TensorBoard.dev histograms support (#3676)
Update README with TensorBoard.dev histograms support (#3676)

Motivation for features / changes
Histograms are launching on TensorBoard.dev.

Technical description of changes
Update documentation to mention histograms support.
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard