Add more format examples to README

[vassilevsky]

Hello 🙃

I had a hard time understanding which formats for specifying periodic tasks are available. I even had to use a step debugger to get to the actual parsing code.

Here I submit my findings. Hope it will be useful to other users of this library.

Currently the README only mentions one-letter time durations. It looked like a step back from ActiveSupport-enabled durations we had in our Clockwork configuration, like every(15.minutes). That prompted my little research.

Maybe the description can be improved further. Let me know 🙂

[coveralls]

Coverage increased (+0.06%) to 97.917% when pulling d6fc012 on vassilevsky:feature/more-formats-in-readme into 1dcb266 on moove-it:master.

[marcelolx]

Thanks @vassilevsky