CB-6491 add CONTRIBUTING.md
CB-6491 add CONTRIBUTING.md

apache/cordova-registry-web,revolunet/cordova-registry-web,rakatyal/Cordova-Registry-Web-Fork,rakatyal/Cordova-Registry-Web-Fork,revolunet/cordova-registry-web,apache/cordova-registry-web,apache/cordova-registry-web,rakatyal/Cordova-Registry-Web-Fork,revolunet/cordova-registry-web