README: please add: is webp/avif are supported

[vitaly-zdanevich]

No description provided.

[vitaly-zdanevich]

Oh I found

Only PNG and PAM are supported

but why, avif is modern and smaller.

[9ary]

Simply put, we can't support every format under the sun.

shotgun is a few years old at this point, back then webp was a fairly new format with little support (and applications still tend to screw it up to this day). AVIF and JPEG-XL weren't even on the horizon.
PNG isn't the best performer, but it's an adequate format with universal support.

If you want to save in a different format, I suggest piping the PAM output into imagemagick or another conversion tool, that's what it's for.