OWNERS: Add documentation

Change-Id: I23538d7cf979c9af0c9c08d44d560afb1bafabb6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/127044
Commit-Queue: Ted Pudlik <tpudlik@google.com>
Reviewed-by: Chad Norvell <chadnorvell@google.com>
diff --git a/OWNERS b/OWNERS
index 7327d4c..d9a1712 100644
--- a/OWNERS
+++ b/OWNERS
@@ -1,3 +1,6 @@
+# OWNERS for all modules. Please see individual modules for their OWNERS files.
+#
+# Note: a Pigweed core team member should be among the OWNERS for each module.
 alizhang@google.com
 amontanez@google.com
 chadnorvell@google.com