Add LICENSE file

We use the PostgreSQL license, lets make it explicit.

Spotted by Jelte Fennema-Nio

Author: mhagander

LICENSE

@@ -0,0 +1,18 @@
+Copyright (c) 2013-2024, PostgreSQL Global Development Group
+
+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 THE POSTGRESQL GLOBAL DEVELOPMENT GROUP 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 THE POSTGESQL GLOBAL DEVELOPMENT GROUP HAS BEEN ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+THE POSTGESQL GLOBAL DEVELOPMENT GROUP 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 THE POSTGESQL GLOBAL DEVELOPMENT GROUP HAS NO OBLIGATIONS TO
+PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.