Add note about config option

Add note explaining what clean_config_d_dir does and why you shouldn't set it
to True.

pillar.example

@@ -1,4 +1,7 @@
 salt:
+  # Set this to true to clean any non-salt-formula managed files out of
+  # /etc/salt/{master,minion}.d ... You really don't want to do this on 2015.2
+  # and up as it'll wipe out important files that Salt relies on.
   clean_config_d_dir: False
 
   # to overwrite map.jinja salt packages