fix: Set license to BSD 2-clause. Update CONTRIBUTING.

[mjang]

Proposed changes

After discussion, updated LICENSE file to match nginx.org (BSD 2-clause simplified license)
Also updated CONTRIBUTING.md for clarity

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and CHANGELOG.md).
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork
• If the change involves:
  • Code
  • Anything that resembles Personally identifying information (PII)
  • URLs
  • Significant new/revised content
    The change will require at least two (2) approvals before merging. One requirement: use placeholders such as <JWT Token> in place of PII

[jputrino]

I'm checking into whether this should be updated to include "F5, Inc". OK to merge as-is, we can always make the change once the question is settled.

[mjang]

Understood. If we do require "F5, Inc". we may also need to update https://github.com/nginx/nginx/blob/master/LICENSE