docs(lisp/dns): Add initial README file

lisp/dns/README.md

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+dns
+===
+
+This library is a DNS-over-HTTPS client for Common Lisp.
+
+The ambition is to transform it into a fully-featured DNS resolver
+instead of piggy-backing on the HTTPS implementation, but ...
+baby-steps!
+
+Note that there is no Common Lisp HTTP client that fully supports the
+HTTP2 protocol at the moment, so you can not expect this library to
+provide equivalent performance to a native DNS resolver (yet).
+
+## API
+
+The API is kept as simple as it can be.
+
+### Types
+
+The types of this library are implemented as several structs that
+support binary (de-)serialisation via [lisp-binary][].
+
+The existing structs are as follows and directly implement the
+corresponding definitions from [RFC 1035][]:
+
+* `dns-header`
+* `dns-question`
+* `dns-rr`
+* `dns-message`
+
+All relevant field accessors for these structs are exported and can be
+used to inspect query results.
+
+### Functions
+
+All lookup functions are of the type `(function (string &key doh-url)
+(dns-message))` and signal a `dns:doh-error` condition for
+unsuccessful requests.
+
+If `:doh-url` is unspecified, Google's public DNS-over-HTTPS servers
+at [dns.google][https://dns.google] will be used.
+
+Currently implemented lookup functions:
+
+* `lookup-a`
+* `lookup-mx`
+* `lookup-txt`
+
+## Example usage
+
+```lisp
+DNS> (dns-message-answer (lookup-a "git.tazj.in."))
+#(#S(DNS-RR
+     :NAME #S(QNAME :START-AT 29 :NAMES #(12))
+     :TYPE A
+     :CLASS 1
+     :TTL 286
+     :RDLENGTH 4
+     :RDATA #(34 98 120 189)))
+```
+
+## TODO
+
+Various things in this library are currently broken because I only
+implemented it to work for my blog setup, but these things will be
+ironed out.
+
+Most importantly, the following needs to be fixed:
+
+* Each qname *fragment* needs to track its offset, not each qname.
+* The RDATA for a TXT record can have multiple counted strings.
+* qnames should be canonicalised after parsing.
+
+[lisp-binary]: https://github.com/j3pic/lisp-binary
+[RFC 1035]: https://tools.ietf.org/html/rfc1035