Add: commit message conventions to DEVELOPERS.md

[anablaz]

Closes #423

Add the link to the commit message conventions of the Accord project.

Changes

• Add the link to the commit message conventions of the Accord project.

Related Issues

• Issue #423

Author Checklist

• [X] Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• [ ] Vital features and changes captured in unit and/or integration tests
• [ ] Commits messages follow AP format
• [ ] Extend the documentation, if necessary
• [X] Merging to master from fork:branchname
• [ ] Manual accessibility test performed
  • [ ] Keyboard-only access, including forms
  • [ ] Contrast at least WCAG Level A
  • [ ] Appropriate labels, alt text, and instructions