Add per-file OWNERS for docs/mte.md

Change-Id: Iff6b738d2a1d0351e1f44e2f1fb7b3200234d4cc
diff --git a/OWNERS b/OWNERS
index 3818b1d..1859b9e 100644
--- a/OWNERS
+++ b/OWNERS
@@ -5,3 +5,5 @@
 danalbert@google.com
 rprichard@google.com
 yabinc@google.com
+
+per-file docs/mte.md=eugenis@google.com,fmayer@google.com,pcc@google.com