docs: contributing documentation

[anastasialanz]

Since this is now a Hacktoberfest project, it's a good idea to have a contribution doc for anyone checking out this repository. I also linked @mdt2's latest Foundry article for reference. I took bits and pieces from some of our other contribution docs.

• https://github.com/sparkbox/logo-experiments/blob/gh-pages/CONTRIBUTING.md
• https://github.com/sparkbox/sparkpress/blob/main/docs/contributing.md
• https://github.com/sparkbox/bouncy-ball/blob/master/contributing.md
• https://github.com/sparkbox/safe-focus/blob/master/CONTRIBUTING.md
• https://github.com/sparkbox/stylelint-config-sparkbox/blob/master/CONTRIBUTING.md