Create PULL_REQUEST_TEMPLATE.md
Create PULL_REQUEST_TEMPLATE.md

Add PULL_REQUEST_TEMPLATE similar to main cBioPortal project

Former-commit-id: d7592db00027d86e55fb45e12a3ac9cb10463f51
alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend,cBioPortal/cbioportal-frontend,alisman/cbioportal-frontend,alisman/cbioportal-frontend,alisman/cbioportal-frontend,cBioPortal/cbioportal-frontend