Add CONTRIBUTING.md

[juancho0202]

Include a ref to Deno's TS style guide: https://deno.land/manual/contributing/style_guide

[juancho0202]

• Include a mention regarding why gets its properties from separate mixins. I'm thinking being explicit that it is not for re-usability of these mixins but rather for testing purposes.
• The choice of yarn as a package manager and the including of .yarn/* files should be mentioned as well. Here are some refs regarding that:
  • https://yarnpkg.com/getting-started/qa#which-files-should-be-gitignored
  • https://yarnpkg.com/features/zero-installs
  • https://yarnpkg.com/getting-started/install

[juancho0202]

About commit conventions: https://www.conventionalcommits.org/en/v1.0.0/