sha2: document migration path (if any) for users of the `Write` trait after v0.11

[aochagavia]

The following code works on 0.10, but fails to compile on 0.11:

let mut hasher = Sha256::new();
serde_json::to_writer(&mut hasher, &some_serializable_object).unwrap();

It seems like Sha256 no longer implements the std::io::Write trait. Is there some cleaner workaround than creating a newtype wrapping Sha256 and implementing Write for it?

[newpavlov]

It's already documented in the digest changelog:

(Removed) io::Write/Read implementations in favor of the digest_io::IoWrapper type

We plan to release the digest-io crate after sha3: RustCrypto/utils#1462

[aochagavia]

All right, thanks!