Add tensorboard docs

[andrewschreiber]

Added basic docs. LMK what you think @richardartoul

When testing with https://github.com/aymericdamien/TensorFlow-Examples/blob/master/examples/4_Utils/tensorboard_basic.py, I ran into a few issues.

1. SDK watcher never detects that tensorflow experiment ended - it goes on forever.

2. Ending the sdk watcher causes the run to complete. (I guess this is hard to avoid)

3. However when you start the sdk watcher again, it creates a new forever run (even though a new experiment hasn't been created)

Let's figure out the issues before merging this.

[richardartoul]

Re 1): I think I looked into this for awhile and there doesn't seem to be a reliable way of detecting this.

Re 2) Yeah basically it has to end or it looks disconnected.

Re 3) There might be a way to implement "resuming" but I think it would be a decent amount of work and involve some caching on disk. I'd rather implement this once people complain and ask for it TBH