Add zhangkelvin@ as owner for release tools scripts

Change-Id: Ic861bc11d6a7d80c0d22a05ce35998753bb60752
diff --git a/tools/releasetools/OWNERS b/tools/releasetools/OWNERS
index 9962836..d0b8627 100644
--- a/tools/releasetools/OWNERS
+++ b/tools/releasetools/OWNERS
@@ -1,6 +1,7 @@
 elsk@google.com
 nhdo@google.com
 xunchang@google.com
+zhangkelvin@google.com
 
 per-file merge_*.py = danielnorman@google.com