added usage example for snippet spinning

misc/spin_utils.sh

@@ -18,3 +18,10 @@ spinsnip(){
 
     rm $tmpR
 }
+
+## usage example
+# echo '
+# > # test report
+# 1+1;
+# ggplot(iris, aes(Sepal.Width) + geom_histogram()
+# ' | spinsnip some_report