Rollup merge of #59172 - varkor:contributing-contributions, r=steveklabnik
Rollup merge of #59172 - varkor:contributing-contributions, r=steveklabnik

Update and clean up several parts of CONTRIBUTING.md

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