Update PRESUBMIT.cfg to include checks for release_notes.

BUG=b/173034990
TEST=manually checking on some changelists.
RELEASE_NOTE=None

Change-Id: I5bc36bb524a3f32a29685dc2a5857b276e87708d
Reviewed-on: https://cos-review.googlesource.com/c/third_party/overlays/portage-stable/+/8620
Reviewed-by: Dexter Rivera <riverade@google.com>
Tested-by: Vaibhav Rustagi <vaibhavrustagi@google.com>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index 43d092e..8bdbe9e 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -5,6 +5,9 @@
 # Make sure Manifest files are valid.
 manifest_check: true
 
+# Make sure RELEASE_NOTE field is present.
+release_note_field_check: true
+
 stray_whitespace_check: false
 long_line_check: false
 cros_license_check: false