Add a usage

Scripts/developer_scripts/git-show-content

@@ -3,7 +3,21 @@
 zmodload zsh/stat
 set -e
 
-git=git
+if [ "x$1" = "x--help" -o "x$1" = "x-h" ]; then
+    cat <<EOF
+Usage: $0
+ [HEAD] [BASE]
+
+List the commits in the range BASE..HEAD, with --first-parent
+and show for each the weight of the commit.
+
+[HEAD] is the head of the branch you want to display.
+Its default value is 'HEAD'.
+
+[BASE] is the base of the range. Its default value is 'cgal/master'.
+EOF
+    exit 0
+fi
 
 commit=${1:-HEAD}
 base=${2:-cgal/master}