add autorelay documentation

p2p/host/relay/doc.go

+/*
+The relay package contains host implementations that automatically
+advertise relay addresses when the presence of NAT is detected. This
+feature is dubbed `autorelay`.
+
+System Components:
+- AutoNATService instances -- see https://github.com/libp2p/go-libp2p-autonat-svc
+- One or more relays, instances of `RelayHost`
+- The autorelayed hosts, instances of `AutoRelayHost`.
+
+How it works:
+- `AutoNATService` instances are instantiated in the
+  bootstrappers (or other well known publicly reachable hosts)
+
+- `RelayHost`s are constructed with
+  `libp2p.New(libp2p.EnableRelay(circuit.OptHop), libp2p.Routing(makeDHT))`.
+  They provide Relay Hop services, and advertise through the DHT
+  in the `/libp2p/relay` namespace
+
+- `AutoRelayHost`s are constructed with `libp2p.New(libp2p.Routing(makeDHT))`
+  They passively discover autonat service instances and test dialability of
+  their listen address set through them.  When the presence of NAT is detected,
+  they discover relays through the DHT, connect to some of them and begin
+  advertising relay addresses.  The new set of addresses is propagated to
+  connected peers through the `identify/push` protocol.
+
+*/
+package relay