chacha20/salsa20/salsa20-core: Improve README.md

[tarcieri]

Each of these crates could use a README.md with some basic information, and the following added to their Cargo.toml files:

readme = "README.md"

Right now they look pretty bare on crates.io, e.g.:

https://crates.io/crates/chacha20

[newpavlov]

Well, most crates in this org do not have per-crate READMEs right now. I think improving in-crate documentation should have a higher priority, as right now it's essentially non-existent for those crates.

Of course eventually it's worth to add READMEs as well, hopefully for all user-facing crates.

[tarcieri]

I think crate-specific READMEs are important because it aids in discoverability through crates.io