Add OWNERS in external/dng_sdk

* Owners are selected from top CL approvals or owners.
  They will be suggested to review/approve future CLs.
* OWNERS files are recognized by the new find-owners plugin, see .md files in
  https://gerrit.googlesource.com/plugins/find-owners/+/master/src/main/resources/Documentation/

Test: build/make/tools/checkowners.py -c -v OWNERS
Change-Id: I70010b9d6a8f07d736808940c75b3a0741f9d5be
diff --git a/OWNERS b/OWNERS
new file mode 100644
index 0000000..b49c3c2
--- /dev/null
+++ b/OWNERS
@@ -0,0 +1,2 @@
+adaubert@google.com
+zalman@google.com