Check for tardiff and add site to README.

When I rad milestone.sh the first time, one error I hit was that I
didn't have tardiff installed (and obviously didn't read the prereqs
in the README).  Note where you can find tardiff in the README and
also check for tardiff to be available as a sanity check before using
it.

Change-Id: Iafcbd173d3cd06af5873686c38af5aa3bd780e16

README.rst

@@ -7,7 +7,9 @@ Prerequisites
 You'll need the following Python modules installed:
  - launchpadlib
 
-similar_tarballs.sh also requires that you have tardiff installed.
+similar_tarballs.sh also requires that you have tardiff installed.  If it's not
+packaged for your distribution, you can find it at
+http://tardiff.coolprojects.org/.
 
 
 repo_tarball_diff.sh

similar_tarballs.sh

@@ -32,6 +32,11 @@ project=$1
 tarball1=$2
 tarball2=$3
 
+if ! which tardiff &>/dev/null ; then
+    read -sn 1 -p "Please install tardiff, then press any key to continue..."
+    echo
+fi
+
 TMPDIR=`mktemp -d`
 wget -q http://tarballs.openstack.org/$project/$project-$tarball1.tar.gz -O $TMPDIR/tarball1.tar.gz
 echo -n "$tarball1 "