Add license file

LICENSE

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+PostgreSQL License
+
+Copyright (c) 2019, The PostgreSQL Global Development Group (PGDG)
+
+Permission to use, copy, modify, and distribute this software and its
+documentation for any purpose, without fee, and without a written agreement is
+hereby granted, provided that the above copyright notice and this paragraph
+and the following two paragraphs appear in all copies.
+
+IN NO EVENT SHALL PGDG BE LIABLE TO ANY PARTY FOR DIRECT, INDIRECT,
+SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING LOST PROFITS, ARISING
+OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN IF PGDG
+HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+PGDG SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS,
+AND PGDG HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, UPDATES,
+ENHANCEMENTS, OR MODIFICATIONS.