Add all-contributors

[RichardLitt]

Right now, the CONTRIBUTING.md doc references all-contributors. In order to set this up, we should enable the all-contributors bot. This seems the easiest way to get it up and running, rather than using a local npm package.json and having people install and run it (as suggested in the current CONTRIBUTING.md, which wouldn't work regardless.)

[nshCore]

Yes we have this running on the main repo's are you able to install it or do you need me to?

[RichardLitt]

I believe you need to enable it. I don't have access to these repositories at the moment.