Add MAINTAINERS.md file

[abelsiqueira]

The MAINTAINERS.md file list the maintainers of the niche files. Also mention the MAINTAINERS.md file in the contributing docs.

Related issues

Closes #281

Checklist

• [x] I am following the contributing guidelines
• [x] Tests are passing
• [!] Lint workflow is passing
• [x] Docs were updated and workflow is passing
• [ ] CHANGELOG.md was updated

[abelsiqueira]

Link checker failure is expected, as file is being added right now.

[abelsiqueira]

Hi @dpo, I'm adding you as maintainer of the Cirrus CI file in this PR. Are you cool with that?