Add a definitive dependency list to the README

[zslayton]

Something about this isn't quite right; we shouldn't require both gcc and clang. I don't want this to block the PR, though, so I'll open an issue to track it.

Originally posted by @zslayton in #13 (comment)

[zslayton]

The ion-cli's dependency on ion-c (and with it the C toolchain) was removed in v0.3.0. All dependencies are now declared in Cargo.toml and handled automatically by cargo.