README: Add subsection on running Clippy as a rustc wrapper
README: Add subsection on running Clippy as a rustc wrapper

This is useful for projects that do not use cargo.

Signed-off-by: Miguel Ojeda <b1a5a4e53a08f19a6a1ffdf06efc0a3c0c374186@kernel.org>

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust