Level up README and related markdown content

[dblock]

Level up and cleanup markdown content.

See .github#8 for details and recommendations, and common-utils for an example.

• README.md
• CONTRIBUTING.md
• DEVELOPER_GUIDE.md
• MAINTAINERS.md
• ADMINS.md
• SECURITY.md
• RELEASING.md
• TESTING.md
• other .md files

[hdhalter]

Hi @dblock, thanks for posting this issue in the documentation repo! Is this a general request to make sure the content in these files is using the right standards and code of conduct? If so, we can certainly review each file. We are in the process of rewriting the readme.md and contributing.md, and there is another issue open for editing the maintainers.md, so we will make sure to follow the standards.