Add the missing listeners guide chapter

While incomplete it's still a pretty good start.

doc/src/guide/listeners.asciidoc

@@ -0,0 +1,115 @@
+[[listeners]]
+== Listeners
+
+A listener is a set of processes that listens on a port for
+new connections. Incoming connections get handled by Cowboy.
+Depending on the connection handshake, one or another protocol
+may be used.
+
+This chapter is specific to Cowboy. Please refer to the
+http://ninenines.eu/docs/en/ranch/1.2/guide/listeners/[Ranch User Guide]
+for more information about listeners.
+
+Cowboy provides two types of listeners: one listening for
+clear TCP connections, and one listening for secure TLS
+connections. Both of them support the HTTP/1.1 and HTTP/2
+protocols.
+
+=== Clear TCP listener
+
+The clear TCP listener will accept connections on the
+given port. A typical HTTP server would listen on port 80.
+Port 80 requires special permissions on most platforms
+however so a common alternative is port 8080.
+
+The following snippet starts listening for connections
+on port 8080:
+
+[source,erlang]
+----
+start(_Type, _Args) ->
+	Dispatch = cowboy_router:compile([
+		{'_', [{"/", hello_handler, []}]}
+	]),
+	{ok, _} = cowboy:start_clear(my_http_listener, 100,
+		[{port, 8080}],
+		#{env => #{dispatch => Dispatch}}
+	),
+	hello_erlang_sup:start_link().
+----
+
+The xref:getting_started[Getting Started] chapter uses a
+clear TCP listener.
+
+Clients connecting to Cowboy on the clear listener port are
+expected to use either HTTP/1.1 or HTTP/2.
+
+Cowboy supports both methods of initiating a clear
+HTTP/2 connection: through the Upgrade mechanism
+(https://tools.ietf.org/html/rfc7540#section-3.2[RFC 7540 3.2])
+or by sending the preface directly
+(https://tools.ietf.org/html/rfc7540#section-3.4[RFC 7540 3.4]).
+
+Compatibility with HTTP/1.0 is provided by Cowboy's HTTP/1.1
+implementation.
+
+=== Secure TLS listener
+
+The secure TLS listener will accept connections on the
+given port. A typical HTTPS server would listen on port 443.
+Port 443 requires special permissions on most platforms
+however so a common alternative is port 8443.
+
+// @todo Make a complete list of restrictions.
+
+The function provided by Cowboy will ensure that the TLS
+options given are following the HTTP/2 RFC with regards
+to security. For example some TLS extensions or ciphers
+may be disabled. This also applies to HTTP/1.1 connections
+on this listener. If this is not desirable, Ranch can be
+used directly to setup a custom listener.
+
+[source,erlang]
+----
+start(_Type, _Args) ->
+	Dispatch = cowboy_router:compile([
+		{'_', [{"/", hello_handler, []}]}
+	]),
+	{ok, _} = cowboy:start_tls(my_http_listener, 100,
+		[
+			{port, 8443},
+			{certfile, "/path/to/certfile"},
+			{keyfile, "/path/to/keyfile"}
+		],
+		#{env => #{dispatch => Dispatch}}
+	),
+	hello_erlang_sup:start_link().
+----
+
+Clients connecting to Cowboy on the secure listener are
+expected to use the ALPN TLS extension to indicate what
+protocols they understand. Cowboy always prefers HTTP/2
+over HTTP/1.1 when both are supported. When neither are
+supported by the client, or when the ALPN extension was
+missing, Cowboy expects HTTP/1.1 to be used.
+
+Cowboy also advertises HTTP/2 support through the older
+NPN TLS extension for compatibility. Note however that
+this support will likely not be enabled by default when
+Cowboy 2.0 gets released.
+
+Compatibility with HTTP/1.0 is provided by Cowboy's HTTP/1.1
+implementation.
+
+=== Protocol configuration
+
+The HTTP/1.1 and HTTP/2 protocols share the same semantics;
+only their framing differs. The first is a text protocol and
+the second a binary protocol.
+
+Cowboy doesn't separate the configuration for HTTP/1.1 and
+HTTP/2. Everything goes into the same map. Many options are
+shared.
+
+// @todo Describe good to know options for both protocols?
+// Maybe do that in separate chapters?