mavericks: update README

If you upgrade to Mavericks you'll get the following error "failed to
create event tap!" when cmd-key-happy starts.  The granularity of
using the accessibility APIs, which is what cmd-key-happy depends on,
is now much finer.  To fix this: Go to "System Preferences", "Security
and Privacy", "Accessibility" and from there you'll need to drag+drop
the applications (via the Finder) into the approved list.