kamailio.cfg: show how to include a local file if exists

- kamailio-local.cfg is attempted to be loaded if exists in the same
  folder, allowing to set defines/paramters inside it without changing
  main kamailio.cfg

etc/kamailio.cfg

@@ -99,6 +99,9 @@
   ALTER TABLE missed_calls ADD COLUMN dst_domain VARCHAR(128) NOT NULL DEFAULT '';
 #!endif
 
+####### Include Local Config If Exists #########
+import_file "kamailio-local.cfg"
+
 ####### Defined Values #########
 
 # *** Value defines - IDs used later in config