Draft appendix section on the background of WAI-ARIA

[mcking65]

The stub section for this content is at: http://w3c.github.io/aria-practices/#ariaBackground

When working on this issue, there is some old content that may be useful in the file aria-practices-DeletedSectionsArchive.html. The relevant section can be seen here: https://rawgit.com/w3c/aria-practices/master/aria-practices-DeletedSectionsArchive.html#ariaBackground