Provide documentation how to get a local mirror.

From: Michael Prokop <mprokop@sipwise.com>

README.txt

@@ -0,0 +1,33 @@
+How to build the local mirror
+-----------------------------
+
+apt-get clean
+apt-get -y --print-uris --reinstall --download-only install $(dpkg --get-selections | awk '{print $1}') | grep "^\'" | cut -d\' -f2 > install.txt
+wget -c --input-file install.txt
+
+Place downloaded .deb files in /srv/mirror/debs/ inside
+the ISO, then execute ./make_reprepro.sh inside /srv/mirror.
+
+./make_reprepro.sh looks like:
+
+  echo "Setting up configuration for reprepro."
+  mkdir -p debian/conf/
+  cat > debian/conf/distributions << EOF
+  Origin: Debian
+  Label: Debian
+  Suite: stable
+  Version: 6.0
+  Codename: squeeze
+  Architectures: amd64 source
+  Components: main contrib non-free
+  Description: Debian Mirror including Sipwise stuff
+  Log: logfile
+  EOF
+  
+  echo "Building local Debian mirror based on packages found in debs."
+  for f in debs/*deb ; do
+    reprepro --silent -b debian includedeb squeeze "$f"
+  done
+
+You can remove the debs from /srv/mirror/debs/ then to
+save space on the resulting ISO.