Add default code reviewers into OWNERS

Bug: 33166666
Test: gerrit uploader
Change-Id: I1f9dc254702b6a61b80ba29b4b14194382124d8b
diff --git a/OWNERS b/OWNERS
new file mode 100644
index 0000000..e1f6012
--- /dev/null
+++ b/OWNERS
@@ -0,0 +1,3 @@
+# Default code reviewers picked from top 3 or more developers.
+# Please update this list if you find better candidates.
+mathewi@google.com