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

[thalurur]

We don't have TESTING.md other files are updated in the linked PR