Add default code reviewers into OWNERS

This library is used by libtextclassifier.

Bug: 33166666
Test: gerrit uploader
Change-Id: If575b8e0d2d9d28a7fd8e5365792088753eda777
diff --git a/OWNERS b/OWNERS
new file mode 100644
index 0000000..466f8a4
--- /dev/null
+++ b/OWNERS
@@ -0,0 +1,4 @@
+# Default code reviewers picked from top 3 or more developers.
+# Please update this list if you find better candidates.
+include platform/external/libtextclassifier:/OWNERS
+agutkin@google.com