Add more documentation about crop_selector to README

@lencioni pointed out that it was hard to understand exactly what this
new property did.
Add YARD about crop_selector, and smoke-test it

@lencioni pointed out that it was hard to understand exactly what this
new property did. I'm adding some YARD documentation that I previously
forgot to add and a simple spec to make sure that the snapshotter
accepts it.

While I was in the Snapshotter file, I broke up a long line that Rubocop
was complaining about.