Improve docs

The README appears to be the only documentation for this software, and doesn't currently cover some key use cases.

(For example, I just tried supercronic for the first time and am seeing the warning "process reaping disabled, not pid 1" logged, but there is no mention in the docs of process reaping, the implications of having it disabled, what to do to enable it, etc. There is an unrelated mention that "Supercronic needs to be PID 1" to enable supercronic to reload crontabs when appropriate signals are delivered to the container, but again, no docs on or example of how to do that.)

Proposing this ticket to track improving the docs, ideally with a comprehensive description of supercronic's features, usage, and configuration available on a dedicated docs site, or at least in the README.

Start by reviewing the README, especially the existing notes about supercronic being PID 1. Document the process-reaping warning, its implications, how to enable it, and an example of running as PID 1; expand coverage to supercronic's features, usage, and configuration on a docs site or in the README.

Written by the indexing model from the issue text.