Document kronos HTTP API

How about something in jest:

"Kronos time is represented as [Coordinated Universal Time](https://en.wikipedia.org/wiki/Coordinated_Universal_Time) since the [epoch](https://en.wikipedia.org/wiki/Unix_time).  It is measured in units of 100s of nanoseconds, or 1e-7 seconds.  We're not proud of our time representation, and in hindsight, we could have picked a few more humane options for it.  We'll get there.  To make up for it, we've created a bunch of [helper functions](../common/src/time.py) for converting to other time representations."