Update image_picker readme

[madsane29]

The image_picker 0.8.4 solved the flutter/plugins#4124 ImagePickerCache problem, but the readme is not updated and still says:

https://pub.dev/packages/image_picker

"On Android, retrieveLostData will only get the last picked image when picking multiple images, see: #84634."

The example still shows only a way to get only one image from cache, not multiple (if multiple images selected before). Would like an update on README so I know how to use the new feature.

[stuartmorgan-g]

/cc @ydag

[ydag]

@BeMacized , I guess there is an open PR already. Can you have a look?

[mvanbeusekom]

Closing this issue as it has been solved in PR flutter/plugins#4336 which has been merged.

[github-actions]

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