This commit adds a skeleton of "Ruby on Rails 6.0 Release Notes".
It isn't a good time to add changelogs' entries to this guide since we can
redo/revert some things till the final release 6.0.
It would be better to do it close to the release.
But we already can add mentions about major features
that have been added to 6.0. I added mention about "Parallel Testing".
