clarify comment at top of Config.hs

There appears to be some confusion -- several people have wanted to edit
Config.hs as was done in the past. This comment probably won't stop that, but
it's a start.

XMonad/Config.hs

@@ -10,8 +10,8 @@
 -- Portability :  portable
 --
 -- This module specifies the default configuration values for xmonad.
--- Users will typically use record syntax to override particular fields
--- they disagree with, in the defaultConfig structure.
+-- Users should not modify this file. Rather, they should provide their
+-- own ~/.xmonad/xmonad.hs that overrides specific fields in defaultConfig.
 --
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