Re: [webpayments] Transition and Adoption (#94)

I would imagine that a custom payment instrument would be a URL, like so:

```js
var supportedInstruments = ["https://thepaymentapp.com/pay"]
```

That URL can host both a payment website at `index.html` and [links to related applications](https://www.w3.org/TR/appmanifest/#application-object-and-its-members) via `manifest.json`. For example:

```json
{
  "related_applications": [
      {
        "platform": "play",
        "url": "https://play.google.com/store/apps/details?id=com.thepaymentapp.app1",
        "id": "com.thepaymentapp.app1"
      }, {
        "platform": "itunes",
        "url": "https://itunes.apple.com/app/thepaymentapp-app1/id123456789",
      }]
 }
```

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https://github.com/w3c/webpayments/issues/94#issuecomment-185497070