Document safety requirements

[Plecra]

It isn't clear what conditions are required to call the Avx2 searcher methods.

[zakcutner]

Hello, thanks for bringing this up! These methods are unsafe since they are annotated with target_feature(enable = "avx2") although hopefully that may not be required soon. I can add a safety section to the docs in the meantime explaining this since that's a good point.

[BurntSushi]

It should be possible to provide a fully safe API using the same techniques that I used in memchr: https://github.com/BurntSushi/rust-memchr/blob/10f11fbeaaec9d1c892be30718604a09b6622e8e/src/x86/mod.rs#L9-L60 --- The only thing you have to give up is that your search function can't be inlined, but that's probably true anyway, since its core implementation is using AVX.

[zakcutner]

That's a really great idea, thanks 👍 We'll probably change around the API a bit at some point (we'd like to support SSE2 without AVX2 for example) so we can definitely add something like that in.