add how to use strengthColors to docs.

[kenthinson]

Hello. I'm trying to change the strengthColors colors. I am unable to do so. Would you be able to add and example to the readme? Thank you.

TweenSequence<Color>([ TweenSequenceItem( weight: 1.0, tween: ColorTween( begin: Theme.of(context).errorColor, end: Theme.of(context).warning, ), ), TweenSequenceItem( weight: 1.0, tween: ColorTween( begin: Theme.of(context).warning, end: Colors.blue, ), ), TweenSequenceItem( weight: 1.0, tween: ColorTween( begin: Colors.blue, end: Theme.of(context) .colorScheme .primary, ), ), ]),