Auto merge of #3919 - matthiaskrgr:readme_3, r=flip1995
Auto merge of #3919 - matthiaskrgr:readme_3, r=flip1995

readme: update

Remove the crates.io badge from the readme.
Installing clippy via crates.io is deprecated and rustup should be used instead, thus we should not promote crates.io here.

Don't have clippy::correctness" lint category in bold letters, I think it looks a little off.

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