document enhancement

[YousefAK009]

hi, thank you for doing this library

I see some people complain about some inaccurate flags, and this library depends on circle-flags lib, and it needs to change or update flags on that library first

for example, Saudi and Iraq flags are inaccurate

for an easy solution and rather than waiting, I can override this package assets by doing this:

I just made packages/circle_flags/assets/svg/ folder:

then in pubspec.yaml I added the following assets:

  assets:
    - packages/circle_flags/assets/svg/

so any library using this library will override the assets to the new one

it will be helpful if this information is added to the documentation of this library

[cedvdb]

feel free to add that to the readme