Add docs in the README for passing options

This is a great gem, but I almost stopped using it because it didn't seem to support passing API options in the function call. I took one last look before moving on to rolling my own option and realized that in fact it did! Unfortunately, this was undocumented, as far as I can tell.

This pull request adds a step for adding options to the `README`, so that it's clear that it supports passing options, and documents how to go about making use of that functionality.