README: add note about subject prefix

Since the mailing list is shared for various of our tools, the
additional prefix helps to keep patches separated for the projects they
should be applied to.

Signed-off-by: Roland Hieber <rhi@pengutronix.de>

Author: rohieb

README.md

@@ -85,3 +85,5 @@ Signed-off-by: Random J Developer <random@developer.example.org>
 description (Git can do this for you when you use `git commit -s`).
 Then send your patches to <oss-tools@pengutronix.de>, or, if you use GitHub,
 open a pull-request on <https://github.com/pengutronix/microcom>.
+If you send patches, please prefix your subject with "[PATCH microcom]" (for
+example, see the `git-config` manpage for the option `format.subjectPrefix`).