add for-await-of desugared example to readme

[shaunlebron]

This may be obvious to everyone involved, but can we include the following desugared example in the readme, if correct?

---

Original example:

for await (const line of readLines(filePath)) {
  console.log(line);
}

Equivalent to:

for (const _ of readLines(filePath)) {
  const line = await _;
  console.log(line);
}

[domenic]

No, this is not correct. See the third paragraph at https://github.com/tc39/proposal-async-iteration#overview-and-motivation