CodingStyle: add information about editor modelines

I recently received a patch including a file that had a vim modeline,
and I realized that nothing specifically proscribed that practice.
Signed-off-by: NJosh Triplett <josh@freedesktop.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/CodingStyle

@@ -738,6 +738,33 @@ need them.  Feel free to peruse that header file to see what else is already
 defined that you shouldn't reproduce in your code.
 
 
+		Chapter 18:  Editor modelines and other cruft
+
+Some editors can interpret configuration information embedded in source files,
+indicated with special markers.  For example, emacs interprets lines marked
+like this:
+
+-*- mode: c -*-
+
+Or like this:
+
+/*
+Local Variables:
+compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
+End:
+*/
+
+Vim interprets markers that look like this:
+
+/* vim:set sw=8 noet */
+
+Do not include any of these in source files.  People have their own personal
+editor configurations, and your source files should not override them.  This
+includes markers for indentation and mode configuration.  People may use their
+own custom mode, or may have some other magic method for making indentation
+work correctly.
+
+
 
 		Appendix I: References