Add a brief description about the regex.

dialplan/external/001_Incoming.xml

@@ -1,5 +1,13 @@
 <include>
   <!--This will transfer you to a local extension given if the call was sent unauthenticated-->
+  <!--
+  This matches all incoming calls matching the following regular expression: ^(10[0-1][0-9])$
+  You can learn the basics of regular expressions here: http://wiki.freeswitch.org/wiki/Regex
+  
+  In brief:
+  this matches all calls that are to 1000 through 1019.
+  The transfer application sends the call to the matching extension on the internal profile.
+  -->
   <exension name="Local_Extensions">
     <condition field="destination_number" expression="^(10[0-1][0-9])$">
       <action application="transfer" data="$1 XML internal" />