Clarify licensing for test data

[kalev]

During the package review for libheif-rs in Fedora, Fabio Valentini noted (https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=2235088#c2) that it's unclear what license applies to the test pictures in data/ directory. Could you add a note somewhere clarifying this, please? Most important for Fedora is that they are under a license that allows redistributing them; otherwise we'd have to roll a custom crate that excludes the files.

Thanks!

[Cykooz]

Is CC BY-SA 4.0 license will be good enough for Fedora?

[kalev]

Thanks! Yes, that's perfect. I think the main concern that Fabio had was that maybe the images are downloaded from somewhere and are actually not redistributable - anything from our https://docs.fedoraproject.org/en-US/legal/allowed-licenses/#_allowed_content_licenses is completely fine.

[Cykooz]

Ok, I'll replace couple of images whose license is unknown to me. After that I'll add a file with license text into "data" directory.

[kalev]

Sounds good. Thanks!

[Cykooz]

I have released v0.21 with updated testing images under CC BY-SA 4.0 license.

[kalev]

Excellent, thank you!