Docs: Add contributors to README

[DirtyF]

Fix #533

All existing 46 contributors were added to the README with the help of all-contributor-cli.

Two new npm scripts are available on development:

• contributors:add to add a new contributor,
• contributors:generate to regenerate the file when changes are made to the .all-contributorsrc config JSON file. (e.g. in case one want to complete with type contributions)

P.S: I opened an issue on the ability to update with all current contributors at once. I thought that was the point of the generate command.

/cc @verythorough

[verythorough]

This looks great! I'm so glad to have it. At some point, we'll want to get this to display on netlifycms.org via the netlify-cms-www repo. I imagine we'll pull directly from the json file and style it up pretty. 😄 Feel free to file an issue for that.

We'll also want to pull more contributors into that list - people writing tutorial blog posts, building template sites, filing detailed bug reports, etc.

Going forward, we'll want to remember to have people add themselves to the list when they make a first time contribution, too. :)

[verythorough]

@Benaiah or @erquhart feel free to merge if everything looks good to you.

[Benaiah]

LGTM