contributing: add section about testing changes
contributing: add section about testing changes

offa/Criterion,offa/Criterion,a1lu/Criterion,Snaipe/Criterion,Snaipe/Criterion,offa/Criterion,Snaipe/Criterion,a1lu/Criterion,offa/Criterion,Snaipe/Criterion,a1lu/Criterion,offa/Criterion,a1lu/Criterion,Snaipe/Criterion,a1lu/Criterion