Add clarification note on code copy/pasting

guidelines/markdown.md

  *  Unless you are pasting *output*, avoid prompt char (`$`, `%`, `%`)
     in order to provide code that can be copied.  To convey the need of
     root privilege, mention that in surrounding text or use *sudo*.
+
+    Note that this does not mean that code *should* be copied around
+    to a productive environment, but in most cases it's good if reader
+    can *try it out* the snippet.  Know your audience and use your best
+    judgment.