APNG is official now

In the readme, I see

APNG is an excellent extension for regular PNG, and it is also very simple to use and not conflicting with current PNG standard (It consists a standard PNG header, so if your platform does not support APNG, it will be recognized as a normal PNG with its first frame being displayed as a static image). But unfortunately, it is a rebel format so that it is not accepted by the PNG group.

This was correct at the time, but APNG is now part of the official PNG standard since October 2022, maintained by the World Wide Web Consortium (W3C). I suggest the following update to that part of the readme:

APNG is an excellent extension for regular PNG, and it is also very simple to use and not conflicting with older PNG implementations (It consists a standard PNG header, so if your platform does not support APNG, it will be recognized as a static PNG with its first frame being displayed as a static image). Happily, APNG is now part of the official PNG standard at W3C

Edit the APNG paragraph in the README, using the linked W3C PNG specification to confirm the October 2022 status. Update the outdated wording while preserving the explanation of fallback behavior; done means the README accurately describes APNG as part of the official standard.