update readme to specify both lint groups are needed for the whole set

update readme to specify both lint groups are needed for the  whole set

As a new user and newbie to rust, after reading this part of the readme("whole set"), I thought I was seeing a bug when I used `#![deny(clippy)]` but also had to add `#![deny(shadow_unrelated)]`.
But this explained it: https://github.com/Manishearth/rust-clippy/blob/3322ffa8a048ef5369d3cdd914869fdf383473a4/src/lib.rs#L108