New WD

Hello IW WG,

Just a note that we've released a new official Working Draft based on the
end-of-April checkpoint: https://www.w3.org/TR/2019/WD-webxr-20190521/.
(We will get more timely as your chairs get more experience at turning this
crank. :)

As always, for working on the evolving future of WebXR, you should continue
to refer to the Editor's draft <https://immersive-web.github.io/webxr/> version
in our repo <https://github.com/immersive-web/webxr> on Github.

-Chris and Ada