Expand the help text in extract-docker.sh

extract-docker.sh

@@ -2,6 +2,10 @@
 
 if [ $# -lt 2 ]; then
     echo $0 image dir
+    echo
+    echo This extracts \'dir\' from the docker image named \'image\' into the
+    echo current directory. NOTE: This removes the existing directory named
+    echo \'dir\' first.
     exit 1
 fi