Auto merge of #5289 - polybuildr:setup, r=matthiaskrgr
Auto merge of #5289 - polybuildr:setup, r=matthiaskrgr

Mention the setup instructions in CONTRIBUTING

Multiple people seem to have missed this based on questions asked in the Discord channel. Might as well mention it up front.

changelog: none

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