Add instructions to add contributors to CONTRIBUTING.md

[emmyft]

Motivation: is your idea related to an existed problem? Please describe.
It's not sustainable to be the only one adding contributors to the list on README.md

Proposal: describe the solution you'd like
Write instructions for adding contributors using the all-contributors bot to CONTRIBUTING.md; the basic steps are here: https://allcontributors.org/docs/en/bot/usage