Add instructions on how to use http2 with node.js enabled appengine

[kenchris]

subject says it all

[JustinBeckwith]

We sorta have support for HTTP/2. We terminate HTTP/2 connections at our load balancer, and then forward HTTP/1.1 requests to your application. We don't quite have HTTP/2 wired the whole way through to your app just yet.

[kenchris]

OK, that would be nice, so that I can avoid vulcanizing my resources and do something similar to https://github.com/GoogleChrome/http2push-gae

[jmdobry]

Moved to GoogleCloudPlatform/community#226