Minor cleanups

Did some minor edits on the readme. 

What do you think about adding a contributing.md with the "Don't post if it's Sass" message, and instructions to test with the `sass` command before posting an issue? Probably few will read it, but could help those that do.

Last thought I had about 6.0 — should we prevent any of the watch or update commands (so `watch`, `update`, and `poll`) from going through? Could prevent some issues.