Update append-outside-loop.md to use string buffer instead of string concatenation

[maschwenk]

From my research, this method is much faster.

[jquerybot]

Thank you for your pull request. It looks like this may be your first contribution to a jQuery Foundation project, if so we need you to sign our Contributor License Agreement (CLA).

📝 Please visit http://contribute.jquery.org/CLA/ to sign.

After you signed, the PR is checked again automatically after a minute. If there's still an issue, please reply here to let us know.

---

If you've already signed our CLA, it's possible your git author information doesn't match your CLA signature (both your name and email have to match), for more information, check the status of your CLA check.

[maschwenk]

I have signed CLA

[scottgonzalez]

Faster, but not much faster. In 99% of user code the difference will be meaningless, so I'd prefer to keep this out of general recommendations.

[AurelioDeRosa]

I'd keep the code as it is as well.

[arthurvr]

Thanks for the suggestion, though!

[maschwenk]

Sure, no problem. Thanks for review. I was coming off trying to optimize an IE8 string concatenation...where this kind of thing unfortunately matters, but agree that it doesn't belong in these docs.