Add contributing.md file

[buzzedword]

Rationale

It's great that you included contribution instructions in your readme. the contributing.md file is a github specific file, which will include your instructions anytime someone wants to submit a patch. For example, on this pull request page. You can read more about it here: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

You can set this file up in three places:

• at the root
• in the docs folder
• in the .github folder

this is all a matter of personal preference. My preference is the .github folder, since that lets me keep all my github specific metadata in one place-- for example, if it later wanted to add github actions.