Add note about useful quote filter.

library/commands/shell

@@ -47,6 +47,9 @@ notes:
       playbooks will follow the trend of using M(command) unless M(shell) is
       explicitly required. When running ad-hoc commands, use your best
       judgement.
+   -  To sanitize any variables passed to the shell module, you should use 
+      "{{ var | quote }}" instead of just "{{ var }}" to make sure they don't include evil things like semicolons.
+
 requirements: [ ]
 author: Michael DeHaan
 '''