Reorganize contributor list in README.
Reorganize contributor list in README.

* Switch to more compact comma-separated list display
* Add sections for creator & maintainers
* Add contributors since previous release

wardi/urwid,inducer/urwid,urwid/urwid,inducer/urwid,inducer/urwid,urwid/urwid,wardi/urwid,urwid/urwid,wardi/urwid