contributing: describe PRs consisting of multiple commits
contributing: describe PRs consisting of multiple commits

Fixes #2855

jjlakis/rkt,Shashwatsh/rkt,joshix/rkt,Wilybald/rkt,sstabellini/rkt,dgonyeo/rkt,euank/rkt,nhlfr/rkt,s-urbaniak/rkt,kinvolk/rkt,shobhit85/rkt,coreos/rkt,jakon89/rkt,yifan-gu/rocket,akshaykarle/rkt,yifan-gu/rkt,akshaykarle/rkt,achanda/rkt,jonboulle/rkt,rkt/rkt,shobhit85/rkt,Wilybald/rkt,lucasallan/rkt,Shashwatsh/rkt,nhlfr/rkt,cab105/rkt,sstabellini/rkt,lucasallan/rkt,sgotti/rkt,alban/rkt,achanda/rkt,dgonyeo/rkt,intelsdi-x/rkt,coreos/rkt,alban/rkt,yifan-gu/rocket,achanda/rkt,rkt/rkt,rkt/rkt,Shashwatsh/rkt,Wilybald/rkt,akshaykarle/rkt,jjlakis/rkt,lucasallan/rkt,yifan-gu/rkt,yifan-gu/rkt,yifan-gu/rocket,s-urbaniak/rkt,jakon89/rkt,jakon89/rkt,Wilybald/rkt,intelsdi-x/rkt,ybubnov/rkt,monstermunchkin/rkt,nhlfr/rkt,jonboulle/rkt,monstermunchkin/rkt,alban/rkt,dgonyeo/rkt,ybubnov/rkt,squeed/rkt,euank/rkt,jonboulle/rkt,euank/rkt,lucasallan/rkt,sgotti/rkt,squeed/rkt,monstermunchkin/rkt,joshix/rkt,joshix/rkt,sstabellini/rkt,rkt/rkt,squeed/rkt,shobhit85/rkt,kinvolk/rkt,joshix/rkt,cab105/rkt,kinvolk/rkt,sgotti/rkt,coreos/rkt,intelsdi-x/rkt,s-urbaniak/rkt,cab105/rkt,ybubnov/rkt,jjlakis/rkt,kinvolk/rkt