Setup TensorBoard-plugin-example with our Greeter plugin

[teamdandelion]

This PR migrates the tutorial from https://github.com/tensorflow/tensorboard/blob/master/ADDING_A_PLUGIN.md into this repository.

The code examples have been copied over, and fixed so that they actually work and produce a functioning "Greeter TensorBoard".

I renamed "tensorboard-x" to "greeter_tensorboard" and moved it to top level. I replaced "my-plugin" with greeter plugin. I moved the tutorial from ADDING_A_PLUGIN into the README. I moved the existing README contents (mostly bazel tips and tricks) into "bazel_tips.md".

[wchargin]

Cool—I'll take a look tomorrow!

[wchargin]

Looks like this got merged instead of squashed:

$ git lg
*   1260987 - (HEAD -> master, origin/master, origin/HEAD) Merge pull request #1 from tensorflow/dandelion-reorg (19 hours ago) <Dandelion Mané>
|\  
| * fa9f792 - Address William's comments (19 hours ago) <Dandelion Mané>
| * 7eafeee - Address chi's comments (21 hours ago) <Dandelion Mané>
| * 4c979f0 - Update the plugin example to have a real working greeter-tensorboard. (2 days ago) <Dandelion Mané>
| * e4d6675 - Rename to greeter_plugin, greeter_tensorboard. Add code from plugin tutorial. (2 days ago) <Dandelion Mané>
|/  
* a2d757d - Create TensorBoard plugin example (#1) (7 days ago) <Justine Tunney>
* 1d9158b - Initialize repository with legal boilerplate (8 days ago) <Justine Tunney>

So:

1. We should configure the repository to disallow merge and default to squash.
2. IMO we should force-push to relinearize the history while we still can.

[wchargin]

In particular, I recommend that we force-push wchargin-please-force-push-me-to-master to master. (Created with git rebase --interactive --root and squashing the last few commits.)

(Edit: This was done; the commit that was pushed is b095516b8070fe3001d428f957990719560d869c.)