Add note about the supported Debian release for building

debian/README.source

@@ -67,7 +67,8 @@ If you want actualy Debian binary or source packages to upload to your
 own repository, you'll need to build them as described here.  We have
 some fancy automatic tools to accomplish this.  The tools allow you to
 start from a very bare Debian image and generate working packages.
-Simply run:
+Building in this manner is only supported on Debian sid, though it
+will generate packages for all supported Debian releases.  Simply run:
 
     ./debian/util.sh build-all -ibn -z9
 
@@ -169,4 +170,4 @@ freeswitch-music-*:
     git clone https://github.com/traviscross/freeswitch-sounds.git
     cd freeswitch-sounds && cat debian/README.source
 
- -- Travis Cross <tc@traviscross.com>, Sun, 23 Feb 2014 18:04:31 +0000
+ -- Travis Cross <tc@traviscross.com>, Sun, 23 Feb 2014 18:08:08 +0000