Added README.md for tier 3

Signed-off-by: Natalia Luzuriaga <[email protected]>

tier3/README.md

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+# Tier 3: Working in Public
+
+## What is a Tier 3 Project?
+
+A **Tier 3** project is an **open collaboration** effort where the work is conducted in public. The project is led by smaller, semi-open teams but encourages **limited external contributions**. The work is typically **open source**, but the direction and maintenance of the project are CMS-led, controlled by a smaller group or team, rather than a large, decentralized community. Tier 3 projects may be public-facing tools, utilities, or websites, where external contributions are welcomed but managed closely by the core team.
+
+### Key Characteristics of a Tier 3 Project:
+- **Collaborative in public**, where the work is open to external stakeholders.
+- Led by a **semi-open team** (often organizational or tool-specific).
+- Accepts **limited contributions from external sources**, typically following specific guidelines.
+- Publicly accessible code and documentation.
+- Maintenance and decision-making are generally led by CMS.
+
+---
+
+## Files for a Tier 3 Project
+
+In a Tier 3 project, there are specific files that are required, recommended, or not recommended as part of the CMS Open Source Program Office's repository hygiene guidelines and standards.
+
+| **File**              | **Requirement** | **Description**                                                                                             |
+|-----------------------|-----------------|-------------------------------------------------------------------------------------------------------------|
+| `LICENSE`             | Mandatory       | Defines the licensing terms under which the project is distributed. |
+| `SECURITY.md`         | Mandatory       | Outlines the agency's security policies, including how to report security issues or vulnerabilities in the code. |
+| `README.md`           | Mandatory       | Provides a comprehensive overview of the project, including its purpose, how to install or use it, and any relevant information for users or developers. |
+| `CONTRIBUTING.md`     | Mandatory       | Offers guidelines for contributing to the project, including code standards, how to submit issues, and creating pull requests. |
+| `MAINTAINERS.md`      | Recommended     | Lists the individuals responsible for maintaining the project as well as reviewing and approving pull requests. |
+| `GOVERNANCE.md`       | Recommended     | Describes the governance model of the project, such as decision-making processes and rules for contributing. It ensures a transparent process for managing the project. |
+| `CODEOWNERS.md`       | Recommended     | Defines ownership of various sections of the repository. |
+| `COMMUNITY_GUIDELINES.md` | Mandatory   | Defines expectations for interactions within the project's community, including how external contributors should engage and behave. |
+| `CODE_OF_CONDUCT.md`  | Mandatory       | Establishes guidelines for acceptable behavior within the community, setting expectations for how contributors should interact in a respectful and collaborative manner. |
+
+For more information about required sections and content within the files above, please visit (maturity-model-tiers.md)[https://github.com/DSACMS/repo-scaffolder/blob/main/maturity-model-tiers.md].