update README re: maxminddb module
update README re: maxminddb module

git-svn-id: f4d3965f88d1c4649b3a56f994ec502c1935526e@26777 83f8c0fa-5cd7-49b8-a9ad-5e47a2db8e72

duo-labs/neustar2mmdb,duo-labs/neustar2mmdb