Move "Contributing" from README to CONTRIBUTING

[JoshuaVandaele]

I'm not submitting a pull request to create a CONTRIBUTING file myself, because it is typically best created by a maintainer who has in-depth knowledge of the project and can provide comprehensive information.

I also recommend reading this article by GitHub on how to effectively communicate contribution guidelines.

(Tagging @Bash-09, since you created the Contributing section in the README)