Add allcontributors spec

[jesstelford]

Give recognition to everyone who's contributing, and what they're contributing via the allContributors spec.

• We can initialise a config with the all-contributors CLI
• Then add the bot
• And mention folks in PRs to add them

It would add this to the README.md file:

[timleslie]

Extra context:

timl [09:49]
How do people feel about the `"author"` field in our `package.json` fields? At the moment it’s more or less whoever was the first person to create the package. I feel like it might be nicer if we credit it as `The Keystone Development Team`, or `Thinkmill`, or something else which don’t link it to a single individual. But maybe the status quo is fine and I’m overthinking things. Thoughts?

jess [10:02]
There's also a `contributors` field

Which can be an array
I also want to add this: https://github.com/keystonejs/keystone-5/issues/759
So, to answer your question: I think `The Keystone Development Team` is a great "author", then we fill out the `contributors` and `.allcontributors.rc` files with everyone's details :+1: