doc: Add github badge for nightly builds (#6009)

Right now only pushes to our master branch have a build status badge.
This adds an additional badge for the nightly scheduled build. See
[here](https://github.com/groszewn/tensorboard/tree/ci_badge#readme) for
what the updated README would look like.

Author: groszewn

README.md

@@ -1,4 +1,4 @@
-# TensorBoard [![GitHub Actions CI](https://github.com/tensorflow/tensorboard/workflows/CI/badge.svg)](https://github.com/tensorflow/tensorboard/actions?query=workflow%3ACI+branch%3Amaster+event%3Apush) [![PyPI](https://badge.fury.io/py/tensorboard.svg)](https://badge.fury.io/py/tensorboard)
+# TensorBoard [![GitHub Actions CI](https://github.com/tensorflow/tensorboard/workflows/CI/badge.svg)](https://github.com/tensorflow/tensorboard/actions?query=workflow%3ACI+branch%3Amaster+event%3Apush) [![GitHub Actions Nightly CI](https://github.com/tensorflow/tensorboard/workflows/nightly-release/badge.svg)](https://github.com/tensorflow/tensorboard/actions?query=workflow%3Anightly-release+branch%3Amaster) [![PyPI](https://badge.fury.io/py/tensorboard.svg)](https://badge.fury.io/py/tensorboard)
 
 TensorBoard is a suite of web applications for inspecting and understanding your
 TensorFlow runs and graphs.