headers: add explicit note against local changes in the README

Even with the step by step guide people sometimes get confused.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/include/drm/README b/include/drm/README
index a50b02c..f3cdf5d 100644
--- a/include/drm/README
+++ b/include/drm/README
@@ -67,6 +67,8 @@
 
 When and how to update these files
 ----------------------------------
+Note: One should not do _any_ changes to the files apart from the steps below.
+
 In order to update the files do the following:
  - Switch to a Linux kernel tree/branch which is not rebased.
 For example: airlied/drm-next