[w3c/manifest] Document the unique identifier or tuple of identifiers for a PWA (#586)

If one is building a PWA Directory or App Store or search engine that detects PWAs one needs a way to uniquely identify a PWA from just the manifest. 

Currently the spec doesn't explicitly say what that identifier or tuple of identifiers should be which leads to issues like: https://github.com/GoogleChrome/gulliver/issues/323

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https://github.com/w3c/manifest/issues/586