[ Observatory ] Add OWNERS file

Change-Id: Ib74c55e70f5f08890d00df9828737b34b6a12585
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434841
Reviewed-by: Alexander Thomas <[email protected]>

Author: bkonyi

pkg/observatory/OWNERS

@@ -0,0 +1,4 @@
+set noparent
+file:/tools/OWNERS_VM
+# In addition to VM team allow global owners.
+file:/OWNERS