If you don't know it, it's hard to figure out the difference between
the linux-headers folder and the include/standard-headers folder.
So let's add a short explanation to clarify the difference.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>