contrib: add some docs for the DNS interface

contributing/README.md

@@ -11,3 +11,4 @@ understanding, and contributing changes to, the Consul codebase.
 2. [Configuration](./checklist-adding-config-fields.md)
 3. [Streaming](./streaming)
 4. [Network Areas](./network-areas)
+5. [Service Discovery](./service-discovery)

contributing/service-discovery/README.md

@@ -0,0 +1,5 @@
+# Service Discovery
+
+This section is still a work in progress.
+
+1. [DNS Interface](./dns.md)

contributing/service-discovery/dns.md

@@ -0,0 +1,28 @@
+# DNS Interface
+
+The DNS interface allows users to find the IP address and port of services, using DNS
+queries. The DNS interface is in many ways similar to an HTTP API. The major difference is
+in the DNS protocol.
+
+There are lots of guides to DNS, the following list is a short reference that should help you
+understand the parts that are relevant to the DNS interface in Consul. Full details about
+the DNS protocol can be found in the RFCs: [RFC 1035], [RFC 6891], [RFC 2782], and others.
+
+[RFC 1035]: https://tools.ietf.org/html/rfc1035
+[RFC 6891]: https://tools.ietf.org/html/rfc6891
+[RFC 2782]: https://tools.ietf.org/html/rfc2782
+
+* [wikipedia: DNS message format](https://en.wikipedia.org/wiki/Domain_Name_System#DNS_message_format)
+  is a quick introduction to the format used for queries and replies
+* [RFC 1035 Section 4.1.1](https://datatracker.ietf.org/doc/html/rfc1035#section-4.1.1)
+  is a good reference for when to use specific response codes and what the different header
+  bits refer to. 
+
+
+## DNS Server
+
+The DNS interface is implemented as a DNS server using [miekg/dns] and the handlers for
+requests are in `agent/dns.go`.
+
+
+[miekg/dns]: https://github.com/miekg/dns