Document `--tsdb` in readme

[goodboy]

Once #308 lands we'll want users to get testing. We need a "storage" section in the readme detailing the deps, sudo req, and cli flag as well as expected behaviour.

TODO:

• [ ] readme section describing pikerd --tsdb and docker deps
• [ ] config options for how much data to load?
• [ ] description of expected behavior and mkts file locations on system