add contribution guidelines
add contribution guidelines

potree/PotreeConverter,potree/PotreeConverter,potree/PotreeConverter,Georepublic/PotreeConverter,potree/PotreeConverter,potree/PotreeConverter,eokeeffe/PotreeConverter,sverhoeven/PotreeConverter,sverhoeven/PotreeConverter,kyroskoh/PotreeConverter,eokeeffe/PotreeConverter,sverhoeven/PotreeConverter,kyroskoh/PotreeConverter,Georepublic/PotreeConverter,eokeeffe/PotreeConverter,kyroskoh/PotreeConverter,potree/PotreeConverter,Georepublic/PotreeConverter,kyroskoh/PotreeConverter,potree/PotreeConverter,Georepublic/PotreeConverter,sverhoeven/PotreeConverter