Document socket activation feature

- Particular mention to included systemd user units

README.md

@@ -266,6 +266,9 @@ when starting your Wayland compositor (i.e. logging in to your
 desktop), and then run `footclient` instead of `foot` whenever you
 want to launch a new terminal.
 
+Foot support socket activation, which means `foot --server` will only be
+started the first time you'll run `footclient`. (systemd user units are
+included, but it can work with other supervision suites).
 
 ## URLs
 

doc/foot.1.scd

@@ -81,7 +81,7 @@ the foot command line
 	Initial working directory for the client application. Default:
 	_CWD of foot_.
 
-*-s*,*--server*[=_PATH_]
+*-s*,*--server*[=_PATH_|_FD_]
 	Run as a server. In this mode, a single foot instance hosts
 	multiple terminals (windows). Use *footclient*(1) to launch new
 	terminals.
@@ -116,6 +116,19 @@ the foot command line
 	*--server-socket* option in *footclient*(1) and point it to your
 	custom socket path.
 
+	If the argument is a number, foot will interpret it as the file descriptor
+	of a socket provided by a supervision daemon (such as systemd or s6), and
+	use that socket as it's own.
+
+	Two systemd units (foot-server@.{service,socket}) are provided to use that
+	feature with systemd. They need to be instantiated with the value of
+	$WAYLAND_DISPLAY (multiples instances can co-exists).
+
+	Note that starting *foot --server* as a systemd service will use
+	the environment of the systemd user instance; thus, if you need specific
+	environment variables, you'll need to import them using *systemctl --user
+	import-environment* or use a drop-in for the foot-server service.
+
 *-H*,*--hold*
 	Remain open after child process exits.