[Community] Update repository guidelines in README

[mkmeral]

Problem Statement

Part of Community Health & Ecosystem Strategy

Goal

We want contributors to understand the path forward before they start. The README should clearly guide them to the right contribution path.

Proposed Solution

Requirements

README Updates

Add prominent notice at the top, along the lines of:

> **Note:** We don't accept external tools to this repository. To publish your own tool, use our [GitHub template](link) and publish to PyPI. Your tool can be listed in our [community catalog](link).
>
> See our [extensibility guide](link) for details on why and how to contribute tools.

CONTRIBUTING.md Updates

• Explain why we don't accept external tools (security, maintenance)
• Link to the template
• Link to the catalog
• Clarify what we DO accept:
  • Bug fixes to existing tools
  • Documentation improvements
  • Performance improvements to existing tools

Acceptance Criteria

• README.md updated with notice
• CONTRIBUTING.md updated with guidance
• Links to template and docs working
• PR merged to tools repo

Dependencies

• GitHub tool template [Community] GitHub Strands Extension Template sdk-python#1537
• Tiered tools explanation [Community] Tiered Tools Explanation docs#464

Use Case

This is critical for all contributors wanting to add a new tool to our tools repository

Alternatives Solutions

No response

Additional Context

No response