Update documentation about 'imageBlot' & image url return

[SDemonUA]

It would be good to know from docs about the necessity of formats: ['imageBlot']. And that resolving upload promise immediately will not work.

[SDemonUA]

Also, I had to do something like this to have same visual effect as in demo import 'quill-image-uploader/dist/quill.imageUploader.min.css';