Mention mailing list for CL reviews in input owners

Probably most people who send CLs to many input owners at once are just
using the Gerrit UI, but let's at least mention the list in INPUT_OWNERS
for those who do go looking there.

Bug: 245989146
Test: none
Flag: EXEMPT only changing OWNERS
Change-Id: If45f18d30efc7bee68cec4885f5632dccf468aa0