README: add reference to submitting-patches

In the hope of improving the quality of patches sent by newcomers,
add a reference to the submitting-patches section of the manual
to the top-level README file.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

README

@@ -21,3 +21,6 @@ Buildroot comes with a basic configuration for a number of boards. Run
 Please feed suggestions, bug reports, insults, and bribes back to the
 buildroot mailing list: buildroot@buildroot.org
 You can also find us on #buildroot on Freenode IRC.
+
+If you would like to contribute patches, please read
+https://buildroot.org/manual.html#submitting-patches