Update CONTRIBUTING.md
Update CONTRIBUTING.md

Placing a "short cut" for the information on how to build & test.
This does not cover all of #2973 but helps with getting up & running quicker.
nunit/nunit,JustinRChou/nunit,appel1/nunit,mikkelbu/nunit,appel1/nunit,mjedrzejek/nunit,OmicronPersei/nunit,JustinRChou/nunit,OmicronPersei/nunit,mikkelbu/nunit,mjedrzejek/nunit,nunit/nunit