Add a comment to recommend sharing design-docs beforehand

TBR=jam@chromium.org

Change-Id: Ifeebeeb58cef6cf10a18dd6ea3a398553c977aac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1784260Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Auto-Submit: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699163}

services/network/OWNERS

+# If you're adding a new feature or modifying non-trivial amount of code
+# in Network Service, please consider sharing the design idea/doc with
+# network-service-dev@chromium.org beforehand.
+
 cduvall@chromium.org
 ericorth@chromium.org
 falken@chromium.org