Expand setup documentation to cover both usage modes

[calebcartwright]

This isn't obvious from the current docs, but users of rusty-hook have two ways of leveraging it in their projects.

1. more automatic/forced way - add it as a dev dependency, and then any dev/contributor on your repo will get the hooks set up automagically
2. more optional/opt-in way - instead of adding it as a dev dep, users could suggest/encourage devs/contributors to manually install (cargo install rusty-hook) and then run the rusty-hook init command if the dev/contributor wanted to set up local git hooks in his or her environment.

We certainly prefer to use option 1 in our personal repos, but given that rusty-hook gives flexibility/choice to the consumers, it's important for the docs to clearly convey those options so folks can choose the option that works best for them and their projects