Update README on the full effects of using the --json option

[Alarich]

From a reply about potential CORS issues when converting a campaign:

one way to avoid this problem with CORS, is to extract the campaign.json file from the exported zip, and convert that using the "use json" option in the converter.. then all the images will be downloaded by the R20Converter app, so CORS becomes irrelevant and all images (as long as they exist) would be downloaded and converted. It just means the conversion will take even more time as it will redownload everything.

The README could be improved with this information.
I've also tested this option and found that, indeed, the files that R20Exporter could not obtain were present and properly downloaded. Even if it does take more time it could be a good fallback option to try for those who used a lot of external assets in their R20 campaigns.