Reoder the README and add a quickstart guide.

The README didn't really present the most relevant information to new
users at the start, it read more like a release notes file. This makes
it a more proper introduction to tinc.

Also add a quickstart guide as a Markdown file in the root of the source
tree, this will make it nicer on GitHub and GitLab, and might help users
that don't want to read the manual.