Write down the reason why we keep a copy of headers to the README file
instead of adding it to every commit messages.

Suggested-by: Jani Nikula <jani.nikula@linux.intel.com>
Original-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Original-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>