Background
The contributing guidelines help potential volunteer contributors to understand where and how they can contribute. This document should align with the current support level for the project (https://www.mediawiki.org/wiki/Readers/Reader_Growth/Maintenance_Levels_and_Responsibilities), the feature-level ownership described in OWNERS.md (if available), and the strategic project goals (if available). For example, if "strategic sunsetting", then the goal is to have no new features and gradually reduce the scope of what this extension is responsible for.
Example of contributing guidelines for an active project: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/ReaderExperiments/+/1204722.
NOTE: Please clarify Reader Growth's support level for the project before creating the contributing guidelines.
Acceptance Criteria
- Add the CONTRIBUTING.md file with consideration for the current support level, feature-level ownership, etc. Use the existing outline described in the EPIC's task description.