Add documentation for OpenAI machine translation

[vlafranca]

Hi,
The configuration exposes settings to use OpenAI to machine translate strings. However there is no documentation explaining how to use it.
Can you add it please ?
I am currently getting a 429 error when trying to use it.

{
    "i18n-ally.translate.openai.apiKey": "sk-mykey",
    "i18n-ally.translate.engines": ["openai"]
}

🌍 Translating "mykey.to.translate" (fr->en)
🌎⚠️ Failed to translate "mykey.to.translate" (fr->en)
🐛 ERROR: Error: Request failed with status code 429
[object Object]
Error: Request failed with status code 429
	at createError (\.vscode\extensions\lokalise.i18n-ally-2.12.0\dist\extension.js:73241:15)
	at settle (\.vscode\extensions\lokalise.i18n-ally-2.12.0\dist\extension.js:73516:12)
	at IncomingMessage.handleStreamEnd (\.vscode\extensions\lokalise.i18n-ally-2.12.0\dist\extension.js:72591:11)
	at IncomingMessage.emit (node:events:531:35)
	at endReadableNT (node:internal/streams/readable:1696:12)
	at process.processTicksAndRejections (node:internal/process/task_queues:82:21)
[object Object]

Thanks in advance