improve documentation of federation

.env.sample

@@ -26,6 +26,10 @@ SECRET_REGISTRATION_VERSION=v1
 # Set "true" to enable federation endpoint on $DOMAIN/.well-known/matrix/server
 SERVE_SERVER_WELLKNOWN=false
 
+# Serve /.well-known/matrix/{server,client} on SERVER_NAME via Traefik.
+# Can be used when SERVER_NAME != DOMAIN and SERVER_NAME is served by Traefik.
+#COMPOSE_FILE="$COMPOSE_FILE:compose.wellknown.yml"
+
 ALLOW_PUBLIC_ROOMS_FEDERATION=false
 
 ## Registration