Add documentation with for-await-of

[ad-m]

I am not sure if tests failing come from my change. Could somebody confirm that?

[RyanZim]

Tests are failing because we have a linter that enforces standard.js code style on code blocks in markdown. Your code style isn't correct, but the linter is having trouble showing the error, hence why it isn't clear from the test output. Correct code style:

for await (const file of klaw('/some/dir')) {
  console.log(file)
}

Please fix, thanks!

[RyanZim]

Merging with broken linter tooling; will fix later.