Add root ruleset.xml for backwards compat
Update README with new install instructions and information about subset standards
Revert "Add root ruleset.xml for backwards compat"

This didn't actually work as intended when the repo was installed in the PHPCS Standards directory.

This reverts commit 664c137b542f8e966ba7d04719b4f1758b81af82.
Merge branch 'folder-structure'