point to backend CONTRIBUTING file
point to backend CONTRIBUTING file

Former-commit-id: 6c48d8fcd57ebff5e035e0a0c467ad9c0af543de
alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,alisman/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend