PSA: publishing new WD of Push API on April 30

[Arthur Barstow]

This is an announcement of the intent to publish a new WD of Push API on 
April 30 using the following document as the basis:

  <https://w3c.github.io/push-api/publish/TR.html>
(The sequence diagram is not found when the above document is loaded but 
the diagram will be available in the WD version.)

If anyone has any major concerns with this proposal, please speak up 
immediately; otherwise the WD will be published as proposed.

-Thanks, ArtB