Document req.route

[bjohansebas]

This is a valid API that we need to document. It's accessible through the public API and should therefore follow semver. See: https://github.com/pillarjs/router/blob/cb2a5fcdf31b9cc92296183ceeff8126b5c57453/index.js#L281

Feel free to open a PR for it. Otherwise, I'll take care of it shortly.

pillarjs/router#196 (comment)