Docs: The proxy option

Thank you for the README. It mentions the proxy option:
```
{
"proxy": {
"worker": "YOUR_WORKER_DOMAIN",
"origin": "YOUR_ORIGIN_DOMAIN"
}
}
```
however it sounds as if this needs to be changed in `wrangler.toml` (it is a route, right?), and after several attempts and dead ends it turned out that this does not work.

Could you change the text to make the proxy option clearer please? E.g.:

```
< You may configure it via the proxy option:
> You may configure it via the proxy option in `config.json`:
```

Many thanks.