Add CONTRIBUTING.md

[adiati98]

Description

Currently, the contributing guidelines are included in each YYYY.md file.

Most OSS contributors are familiar with finding contributing guidelines in the CONTRIBUTING.md or README file. In the long run, this would potentially add an unnecessary task for maintainers to redirect folks to the YYYY.md file.

Suggestions

• Move the guidelines from YYYY.md to CONTRIBUTING.md.
• Dedicate YYYY.md file only to add contributions under "Ongoing contributions YYYY".

[RCheesley]

Fine with me!