updated README.md with how to get geolocation db
updated README.md with how to get geolocation db

lianke123321/centinel-server,ben-jones/centinel-server,iclab/centinel-server,iclab/centinel-server,rpanah/centinel-server,rpanah/centinel-server,rpanah/centinel-server,ben-jones/centinel-server,iclab/centinel-server,ben-jones/centinel-server,lianke123321/centinel-server,lianke123321/centinel-server