[push-api] Add applicability statement (#179)

Paul Banks notes that we aren't very clear about what Push is good for.  Something like this is probably a good idea:

> The Push API is intended for infrequent messages.  If you have a page
> open to site, it might still be suitable to rely on push for messages
> that are infrequent or unpredictable.  However, if you are actively
> communicating with your site, it is best to use more direct means of
> sending messages, such as HTTPS.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/push-api/issues/179