docs/manual: mention checkpackageignore file

Currently, the manual doesn't mention the checkpackageignore file at
all, leaving the user under the assumption that running "make
check-package" and running utils/check-package directly on the package
they have touched would be equivalent, when the former evaluates the
checkpackageignore file while the latter does not. This could cause them
to forget to remove stale entries from that file.

Point out this difference in behavior.

Signed-off-by: Florian Larysch <fl@n621.de>
[Arnout: add that stale lines should be removed from
.checkpackageignore]
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>

docs/manual/contribute.adoc

@@ -321,9 +321,12 @@ 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.
+This command (unlike calling +utils/check-package+ directly) will also warn
+about any stale entries in the +.checkpackageignore+ file that your patches
+might have resolved. If this is the case, make sure to remove those lines
+from +.checkpackageignore+ as well.
 
-To generate it, run:
+Now, you are ready to generate then submit your patch set. To generate it, run:
 
 ----
 $ git format-patch -M -n -s -o outgoing origin/master