[w3c/manifest] Add info push notifications to extensions registry (#529)

BACKGROUND
Looking for manifest files of popular websites, I noticed [quora.com's manifest](https://qsf.ec.quoracdn.net/-jsons.desktop_push_manifest.jsoncc40e313a4597c57.json) includes the following:
``` json
{
  "name": "Desktop Push Notifications",
  "gcm_sender_id": "602221294976"
}
```

I suspect gcm_sender_id may be a vendor-specific extension possibly documented at: https://developers.google.com/cloud-messaging/chrome/client

If that is the case, I recommend we document it in the Web App Manifest spec's Extensions Registry at: https://github.com/w3c/manifest/wiki/Extensions-Registry

/cc @mounirlamouri who may know more here

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/529