Add "All Contributors" listing to repo & docs

[verythorough]

Following the model of Kent C. Dodd's https://github.com/kentcdodds/all-contributors, I'd like to recognize contributions to Netlify CMS, in all of their various forms.

Implementation details are in the link above, and mostly involve markdown changes to the README, along with research to gather all of the contributors to include initially.

[DirtyF]

@verythorough Working on a PR, I went with defaults options, is it OK?

[verythorough]

I think that's awesome for getting us started. Thanks so much!

[tech4him1]

How does the auto-commit work?

[DirtyF]

@tech4him1 I removed it for now, cause I had to add manually each contributor and it was generating a commit for each.