Progress bar for DeepL PHP API

[migliori]

Hello,

I've implemented a progress bar for uploading and translating documents using the DeepL PHP API. I use Javascript Fetch(), $translator->uploadDocument() and $translator->waitUntilDocumentTranslationComplete($document_handle)

The current documentation isn't very detailed, so I'd like to send you a pull-request, which would add an examples directory and the example usage file with progress bar.

I thought it might help other users; if you're interested, please let me know.