Add contribution guidance (CONTRIBUTING.md)

[dgarske]

Description

Adds a CONTRIBUTING.md to the repository root. None of the wolfSSL repositories had one, so our contributor agreement requirement was only discovered after someone had already done the work. See wolfSSL/wolfssh#1146, where an outside contributor withdrew a PR over it and asked us to document the requirement up front.

The same repo-agnostic file is being added to wolfSSL, wolfSSH, wolfMQTT, wolfTPM, wolfBoot, wolfPKCS11, wolfHSM, wolfProvider, wolfSentry and wolfCLU. It covers how to contribute, the contributor agreement and how to request one, our CI (GitHub Actions is public, Jenkins failures get pasted in by a maintainer), coding expectations, and where to report security issues. Please read the file itself rather than a summary here.

Testing

Documentation only - no code changes.

[Copilot]

Pull request overview

Warning

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Adds repository-level contribution guidance to reduce surprise around the required contributor agreement and to document contribution/CI/security expectations up front.

Changes:

• Introduces a new root CONTRIBUTING.md covering contribution paths, contributor agreement steps, CI visibility, coding expectations, and security reporting.

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[dgarske]

Jenkins retest this please