Create CONTRIBUTING.md file

[s-peryt]

At the moment this file is completely missing and there is no guidance for community what can be done. Good learning material how to start with such file can be found here - https://mozillascience.github.io/working-open-workshop/contributing/

[jalaln06]

I want to work on that issue.
@MaldoAlberto
Is there anything specific I should mention?
Is there something you want to highlight here, or I can just get creative, and then we will edit together?