documentation/README: Refer to top-level README for contributions

This may help anyone looking for patch contribution guidelines in the
documentation directory itself.

(From yocto-docs rev: 276740dd3780b0bdf04996efeed3c237fd26da1f)

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/README

@@ -325,3 +325,9 @@ References to the bitbake manual can be done like this:
   See the ":ref:`-D <bitbake:bitbake-user-manual/bitbake-user-manual-intro:usage and syntax>`" option
 or
   :term:`bitbake:BB_NUMBER_PARSE_THREADS`
+
+Submitting documentation changes
+================================
+
+Please see the top level README file in this repository for details of where
+to send patches.