Include AVIF @ README

[xerc]

fyi/ AVIF seems smaller then WEBP therefore higher priority

& you may also .. and drop mod_headers

- L,T=image\/\w+
+ END

[xerc]

ps/ git rm -f Configuration/.htaccess Resources/Private/.gitkeep

[outdoorsman]

So what's the status on this? Would be nice to have the docs up to date even if you don't accept this pull request from a year ago.