Add comparison to at(1) and batch(1)

[dbohdan]

First, thanks for writing this tool! It's an elegant design.

I think people who find it for the first time may wonder how it's different from POSIX at and batch. It could be useful to preempt their questions in the readme. Somebody asked on Hacker News, and I posted a five-point comparison based on my best understanding of both nq and at/batch. Feel free to use the text of that comment under your choice of license.

[leahneukirchen]

Thanks, I added that with minor editing.