Add all-contributors bot

[JalilArfaoui]

https://allcontributors.org/docs/en/bot/overview

[lutangar]

Loving it! I discretely created an issue referencing all things we could do to make our repositories more appealing :octocat:

We could reference it there: https://github.com/dis-moi/.github/issues/3

[MaartenLMEM]

Questions to help prioritization :

• Does it presents only external contributors or also DisMoi devs ?

Currently :

• How many external contributors posted an issue ? Do we know these guys ?
• How many pushed some code ?

More general question related to this issue and some items in https://github.com/dis-moi/.github/issues/3 :

• What can we expect in termes of contribution from this before octobre 2021 ?