Provide documentation to run from command line

[kevinwuhoo]

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.

[mathieudutour]

Yeah happy to accept that.

But I'm working on a new project which is going to make this obsolete soon: http://kactus.io (that's why I've been neglecting this repo a bit lately) so I'd much appreciate contribution there :D