Add documentation about the SrvRecords support in apt

Git-Dch: ignore

diff --git a/README.SrvRecords b/README.SrvRecords
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+DNS SRV record support in apt
+=============================
+
+Apt supports a subset of the DNS SRV server records protocol as
+described in RFC 2782 for service discovery.
+
+Before connecting to the requested server APT will send a SRV
+record request of the form "_$protocol._tcp._$host", e.g.
+"_http._tcp.ftp.debian.org" or "_http._tcp.security.debian.org".
+
+If the server sends SRV records
+as a reply APT will use those to connect to the server(s) in
+this reply. It will honor the "priority" field in the reply.
+
+However it does not implement the "weight" alogrithm as described
+in RFC 2782. It will use an equal weight for each server of the
+same priority.
+
+If connecting to a server fails APT will retry with the next one
+and remove the server from the list of valid servers for this
+session.
+