[backend] document the BSConfig:sibling* and partition settings

src/backend/BSConfig.pm.template

@@ -188,6 +188,27 @@ our $relsync_pool = {
 # Use old style source service handling
 # our $old_style_services = 1;
 
+###
+# Optional support to split the binary backend. This can be used on large servers
+# to seperate projects for better scalability.
+# There is still just one source server, but there can be multiple servers which
+# run each repserver, schedulers, dispatcher, warden and publisher
+#
+# This repo service is the 'home' server for all home:* projects. This and the
+# $reposerver setting must be different on the binary backend servers.
+# our $sibling = 'home';
+#
+# this defines how the projects are split. All home: projects are hosted
+# on an own server in this example
+# our $partition = [ 'home:' => 'home',
+#                    '.*' => 'main',
+#                 ];
+#
+# our $siblings = { 'home' => 'http://home-backend-server:5252',
+#                   'main' => 'http://main-backend-server:5252',
+#                  };
+
+
 # host specific configs
 my $hostconfig = "bsconfig." . Net::Domain::hostname();
 if(-r $hostconfig) {