docs/manual: check-package before submitting patch

Add 'utils/docker-run make check-package' to the default workflow of
submitting patches, just after the rebase and before using format-patch.

Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/contribute.txt

@@ -294,6 +294,12 @@ $ git fetch --all --tags
 $ git rebase origin/master
 ---------------------
 
+Now check the coding style for the changes you committed:
+
+---------------------
+$ utils/docker-run make check-package
+---------------------
+
 Now, you are ready to generate then submit your patch set.
 
 To generate it, run: