doc: Add github badge for nightly builds (#6009)
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.
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard