Add some clarification about other cargo-clippy commands in the README

[BenoitZugmeyer]

Your project is the first found on google, but two other cargo-clippy binaries exists, and (as a cargo beginner) I had a hard time to figure out which is which. Not to undermine your work, but could you add a word in the README to clarify this situation? It looks like I'm not the only one confused about this: #14 .

The other two commands are:

• The crate cargo-clippy, which is the one installed when running cargo install cargo-clippy
• The binary provided by rust-clippy, the official clippy project, which is the one installed when running cargo install clippy