Create CODE_OWNERS.TXT

Author: thewillyhuman

CODE_OWNERS.TXT

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+This file is a list of the people responsible for ensuring that patches for a
+particular part of Swift are reviewed, either by themselves 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),
+GitHub account (G), description (D).
+
+N: Facundo Colunga, Guillermo
+E: uo236856@uniovi.es
+G: thewilly
+D: Data Structures