chore: create contributing guide

[moonmeister]

Create a CONTRIBUTING.md file as a place for new and prospective contributors.

Some examples:

• https://github.com/withastro/astro/blob/main/CONTRIBUTING.md
• https://docs.github.com/articles/setting-guidelines-for-repository-contributors/
• https://www.gatsbyjs.com/contributing/
• https://nuxt.com/docs/community/contribution

[ahuseyn]

PR: #114