ARIA read me to do's

[Michiel Bijl]

There was a list with to do's in the ARIA spec’s README file. I’ve moved these to separate GitHub issues <https://github.com/w3c/aria/labels/documentation>. I can’t really make sense of most, could someone add some background information on these so people less familiar—e.g. me—with the issues can work on them?

—Michiel