README: Add goacmedns, a Go acme-dns client library

[cpu]

👋 Hi @joohoi,

In the process of writing an ACME-DNS hook for a Go ACME client (github.com/xenolf/lego) I ported your pyacmedns library to Go: https://github.com/cpu/goacmedns

This PR adds a link to the README next to pyacmedns. I hope someone finds it useful!

[coveralls]

Coverage remained the same at 92.244% when pulling efeb5eb on cpu:cpu-add-goacmedns into 0bbbf5e on joohoi:master.

[cpu]

My github.com/xenolf/lego ACME-DNS hook using goacmedns is still a WIP (needs tests and some cleanup) but its far enough along that I was able to use it to issue a certificate from the Let's Encrypt ACMEv2 staging environment w/ Lego and ACME-DNS 🎉

[cpu]

For reference, I opened a PR with my initial Lego hook built on this library: go-acme/lego#591

[joohoi]

Yay! This is awesome! Thanks!