Add code owners.

CODE_OWNERS.TXT

@@ -0,0 +1,13 @@
+This file is a list of the people responsible for ensuring that patches for a
+particular part of Swift are reviewed, either by themself or by someone else.
+They are also the gatekeepers for their part of Swift, with the final word on
+what goes in or not.
+
+The list is sorted by surname and formatted to allow easy grepping and
+beautification by scripts.  The fields are: name (N), email (E), web-address
+(W), PGP key ID and fingerprint (P), description (D), and snail-mail address
+(S).
+
+N: Daniel Dunbar
+E: daniel_dunbar@apple.com
+D: Everything in llbuild not covered by someone else