Restructure readme file, document manual testing, various updates

[gudmdharalds]

This pull request restructures the README.md file, moving sections of it into separate files, and adding a new file about versioning, testing and deployment. The updated README.md file refers to these new files and still includes the relevant headings. Various sections of the documentation is updated along the way.

TODO:

• Break README.md file up, move content to new files.
• Add new file about versioning, testing and deployment.
• Move section about manual testing away from RELEASING.md into TESTING.md.
• Add script for manual testing to repository and document in TESTING.md.
• [N/A] Add/update tests -- unit and/or integrated (if needed)
• [N/A] Add to, or update, Scan run detail report as applicable
• [N/A] Check status of automated tests
  • Changes to documentation and added support script only. No impact on tests.
• [N/A] Ensure PHPDoc comments are up to date for functions added or altered
• Changelog entry (for VIP) [ Changelog for version 1.3.4 #333 ]

[wpcomvip-vipgoci-bot]

Auto-approved pull request #341 as it contains only auto-approvable files -- non-functional changes to PHP files or file-types that are auto-approvable (css, csv, eot, gif, gz, ico, ini, jpeg, jpg, json, less, map, md, mdown, mo, mp4, otf, pcss, pdf, po, pot, png, sass, scss, styl, ttf, txt, woff, woff2, yml).

Dismissing obsolete review; not approved any longer