headers: Add README file

Since we're trying to standardise and make things more consistent in
the area, add a basic README which covers some of the more popular
topics.

v2:
 - Drop drm-misc (Daniel Vetter)
v3:
 - Elaborate on when and which headers to update
 - Add a list of headers and the respective "issues"
 - Add file to EXTRA_DIST

Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/Makefile.am b/Makefile.am
index 630edc4..2e46bde 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -133,6 +133,7 @@
 klibdrminclude_HEADERS += $(LIBDRM_INCLUDE_VMWGFX_H_FILES)
 endif
 
+EXTRA_DIST = include/drm/README
 
 copy-headers :
 	cp -r $(kernel_source)/include/uapi/drm/*.h $(top_srcdir)/include/drm/