Document the reason for pinning old version of pillow_heif

[patrakov]

ART-imageio pins the old version of pillow_heif, as newer ones apparently don't work for HDR. But cybersecurity auditors don't know that and barf at anything pinned to an old version. Due to this pin, Arch AUR packages do not include the HEIF saving functionality.

Please add a comment stating the rationale for the pin, and whether it is a genuine bug in newer versions of pillow_heif or ART-imageio should be ported.

[night199uk]

is it true that pillow_heif > e.g. 0.22.0 doesn't work for HDR? what are the issues?
i just raised #2 which fixed all the deprecations and allows using later versions of pillow_heif.

[agriggio]

Hi,
Maybe things have changed. Thanks for the pr, I will check it as soon as I have the chance