Add OWNERS in external/boringssl

* 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: Ifd6cf9e93c2c756dd3d7d80dca6a2cb15f91778f
diff --git a/OWNERS b/OWNERS
new file mode 100644
index 0000000..0fb4b65
--- /dev/null
+++ b/OWNERS
@@ -0,0 +1,2 @@
+flooey@google.com
+kroot@google.com