Add all-contributors bot

[ericmjl]

We should ensure that all contributions are recognized. Following the all contributors spec, we should use the bot to help recognize contributors of all kinds to the project.

https://allcontributors.org/

[ericmjl]

@all-contributors init

[allcontributors]

@ericmjl

I could not determine your intention.

Basic usage: @all-contributors please add @Someone for code, doc and infra

For other usages see the documentation

[ericmjl]

@all-contributors please add @ndousis for code

[allcontributors]

@ericmjl

I've put up a pull request to add @ndousis! 🎉

[ericmjl]

@all-contributors please add @andrewgiessel and @maxasauruswall for code

[allcontributors]

@ericmjl

I've put up a pull request to add @andrewgiessel! 🎉

[ericmjl]

@all-contributors please add @maxasauruswall for code and docs

[allcontributors]

@ericmjl

I've put up a pull request to add @maxasauruswall! 🎉

[ericmjl]

@all-contributors please add @ericmjl for code, docs

[allcontributors]

@ericmjl

I've put up a pull request to add @ericmjl! 🎉

[ericmjl]

@all-contributors add @MihirMetkar for ideas, code