docs: update CONTRIBUTING.md
docs: update CONTRIBUTING.md

Problem / Solution

Last week we updated Finagle's CONTRIBUTING.md and now we want to
apply those changes to our other projects: util, finatra,
twitter-server, scrooge.

RB_ID=915195

koshelev/finagle,twitter/finagle,koshelev/finagle,koshelev/finagle,mkhq/finagle,luciferous/finagle,mkhq/finagle,mkhq/finagle,twitter/finagle,koshelev/finagle,twitter/finagle,mkhq/finagle,twitter/finagle,twitter/finagle,koshelev/finagle,luciferous/finagle,mkhq/finagle,luciferous/finagle,koshelev/finagle,luciferous/finagle,luciferous/finagle,luciferous/finagle,mkhq/finagle