[Misc] Add CONTRIBUTING.md with reference to developer docs

Description

This MR adds a basic CONTRIBUTING.md file to the project root. The aim of the file is to follow the standardised way of telling developers how to contribute (i.e. have a single point of entry that is common throughout most repos). At the same time we want to prevent duplicating information that can potentially go out of sync. As such, this file is kept short and links to the dev docs for the details.

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See this link

References

Closes #1258 (closed)