Document the experimental Golang DNS server

This is mostly for me because I forget how I wrote it (e.g. what happens
when you query a `TXT` record).

README.md

@@ -22,3 +22,49 @@ DOMAIN=sslip.io rspec --format documentation --color spec
   configuration in YAML format for use with [BOSH](https://bosh.io). The
   `pdns_pipe` key is the pipe backend script, and `pdns_pipe_conf` is its
   configuration file.
+
+## Golang DNS Server
+
+An experimental bare-bones DNS server written in Golang is available.
+
+This Golang server is currently not configurable:
+
+- it binds to port 53 (you can't change it)
+- it only binds to UDP (no TCP, sorry)
+- if the hostname queried doesn't match, it doesn't return an _Answer_ section;
+  instead, it returns an _Authorities_ section with an SOA.
+- The SOA record is hard-coded (e.g. _Serial_ is `2020090400`) with the
+  exception of the _NS_ record, which is set to the queried hostname (e.g. `dig
+  big.apple.com @localhost` would have an SOA with an _NS_ record of
+  `big.apple.com.`.
+- The NS, MX records are hard-coded.
+- There are no TXT records, or SRV. If those records (or any other unknown ones)
+  are queried, the server returns no _Answers_ but an _Authorities_ section with
+  the SOA
+
+To run the unit tests:
+```
+cd src
+go get github.com/onsi/ginkgo/ginkgo
+go get github.com/onsi/gomega/...
+ginkgo -r .
+```
+
+To run the server on, say, a Mac, you must first start the server:
+```
+cd src
+go run main.go
+```
+And then, in another window, run a query, e.g.:
+```
+dig +short 127.0.0.1.sslip.io @localhost
+```
+Which will return the expected IP address:
+```
+127.0.0.1
+```
+You will also see a log message in the server window, something similar to the
+following:
+```
+2020/11/22 03:45:44 ::1.62302 TypeA 127.0.0.1.sslip.io. ? 127.0.0.1
+```