switch license to GPL (#17)

* license modifications under GPL

Author: gsmolk

LICENSE.postgresql

@@ -0,0 +1,22 @@
+Portions Copyright (c) 2015-2023, Postgres Professional
+Portions Copyright (c) 2009-2013, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
+
+Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+Portions Copyright (c) 1994, The Regents of the University of California
+
+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 POSTGRES PROFESSIONAL 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 POSTGRES PROFESSIONAL HAS BEEN ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+POSTGRES PROFESSIONAL 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 POSTGRES PROFESSIONAL HAS NO OBLIGATIONS TO
+PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.

README.md

@@ -83,7 +83,7 @@ cd <path_to_PostgreSQL_source_tree> && git clone https://github.com/pgtoolz/pg_b
 
 ## License
 
-This module available under the [GPLv3 license](LICENSE).
+This module is available under the [license](LICENSE.postgresql) similar to [PostgreSQL](https://www.postgresql.org/about/license/), but all modification since 18 July 2023 are available under the [GPLv3 license](LICENSE).
 
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