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.

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

[trombik]

@dblock I think I need SECURITY.md now. would you commit it? or tell me who should I contact. thanks.

[trombik]

@dblock figured it out. https://aws.amazon.com/security/vulnerability-reporting/

[dblock]

All templates can be found in https://github.com/opensearch-project/.github

[VachaShah]

Documentation has been updated recently in PR #85