dartdoc should allow README.md's location to be specified in dartdoc_options.yaml

[jcollins-g]

As @tvolkert pointed out in flutter/flutter#16080, it's rather confusing in flutter to require that a README.md containing top-level information must be so far down in the flutter repo.

[tvolkert]

Ideally it wouldn't even have to be a README.md, since GitHub renders those when you navigate to the directory that contains the file. It'd be great if it could be specified as an arbitrary .md file in dartdoc_options.yaml