Provide documentation to run from command line

Sometimes I want to run this from the command line. It usually happens when I don't commit to the repo constantly and then I want to generate pretty diffs for a bunch of sketch files at once. This could also potentially allow people to use it as a pre-commit hook.

If you think it's useful, I can take a crack at adding some documentation into `docs/` and try to get it to read from the preferences files if added.

Review the existing docs/ content and the command-line workflow described in the issue. Document how to generate diffs for multiple Sketch files, including any preferences-file setup and possible pre-commit use; done means a newcomer can follow the documented steps.

Written by the indexing model from the issue text.