PSA: Martin Thomson joins Push API Editor team

[Arthur Barstow]

Hi All,

We are pleased to announce Mozilla's Martin Thomson has agreed to be a 
co-Editor of the Push API spec <https://w3c.github.io/push-api/>.

Eduardo and Bryan are no longer able to continue as active editors and 
we thank them for their past contributions and we hope their active 
participation will resume.

-Cheers, Team WebApps