[e] (0) Add a section encouraging user agents (especially ATs) to expose the outline and headings
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=25394
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8698 340c8d12-0b0e-0410-8428-c7bf67bfef74
Merge remote branch 'origin/master' into whatwg