Update CONTRIBUTING.md
Update CONTRIBUTING.md

added a part on "no new features w/o tests", closes issue #168
openSNP/snpr,philippbayer/snpr,vivekiitkgp/snpr,philippbayer/snpr,openSNP/snpr,gedankenstuecke/snpr,openSNP/snpr,vivekiitkgp/snpr,openSNP/snpr,philippbayer/snpr,philippbayer/snpr,gedankenstuecke/snpr,gedankenstuecke/snpr,gedankenstuecke/snpr,vivekiitkgp/snpr,vivekiitkgp/snpr