document how and when subprocesses send coverage to nyc & that coverage will be lost for SIGKILLed processes

It seems obvious in retrospect, but I spent hours trying to figure out why I wasn't getting coverage for certain files until I realized it was because I was killing subprocesses that ran those files with `SIGKILL`. Documentation of this in the README would help others with the same situation figure out why quickly, and I'd be happy to make a PR.

It would be nice to get a high-level overview before digging through the code though. Do subprocesses send coverage to nyc in a `process.on('exit', ...)` hook, or where? And do they send it via IPC, some kind of sockets, or some other mechanism? If its a coverage channel that can possibly get blocked it would be helpful to document so that people can troubleshoot why files aren't getting covered.