Example to create country-city info mmdb from a csv file

[Sirius-Aabhas]

For my usecase , I needed to create a mmdb file from scratch with only country level info. But, I found the asn writer example to be not very helpful as I needed only country info. Only after going through the whole blogpost, was I able to understand the creation flow. So, adding this example here to benefit others.

[oschwald]

The failures appear unrelated. The golangci-lint ones appear to be due to a new release of that, which will need to be addressed in a separate Pr.