ReadMe update - using GFPGAN reduces available memory

[AndrWeisR]

Thanks for a fantastic project.

It might be worth mentioning in the readme about GFPGAN Face Correction that if you enable it, it consumes some memory and you might get OOM errors on image dimensions that used to generated before. I experienced that with my 6 GB card, and ended up disabling GFPGAN to make more memory available.