Recommended password hashing algorithms

[tarcieri]

Following up from RustCrypto/meta#10, this is an issue for discussion potentially adding "recommended" badges to certain algorithms in this repo:

[tarcieri]

I'd vote for recommending argon2 and scrypt.

I think it's worth considering NOT recommending sha-crypt, as it does a poor job of resisting cracking attacks compared to all of the others. Its main use should be legacy interop with libcrypt.