Document alternatives

[workingjubilee]

Pursuant to these remarks:

As a maintainer, I would prefer to keep this repo writeable for now so we can apply updates and fixes to benefit the few people still using this library. However, I do think that we should do what we can to encourage most people not to use this library. Perhaps that should include a RustSec advisory so tools like cargo-audit will complain. Documentation pointing to alternatives is definitely a good idea.

One obvious alternative is the image crate but it may have lackluster performance by comparison on a few metrics. That should probably be fixed, but that's an issue for that crate. Are there any other reasonable-to-high-quality alternatives?