README: tweak documentation section.
tap-readme: tweak the format.
Migrate some formula with hard-X11 dependencies.

We have the homebrew-x11 tap for this reason so let's migrate everything
without any reverse dependencies. Ideally I'd also like to migrate
everything that's :recommended and the GTK formulae through there too
(or to a dedicated homebrew-gtk).
Merge remote-tracking branch 'upstream/master'