Declared the project to be FOSS under Postgres license

The history of this project is documented under the Credits section in
the index_adviser/README.index_adviser file.

The original patch was submitted by Kai-Uwe Sattler to the community
mailing lists [1]. And the subsequent patches were also submitted to the
community mailing lists.

Per Postgres project's policy [2], any patches submitted to the
community mailing lists automatically fall under the Postgres license.
Hence, this project is being declared Free and Open Source Software
released under the Postgres license.

[1]: https://www.postgresql.org/message-id/2261C794-2772-42A9-88F5-F05926FD187B%40tu-ilmenau.de
[2]: https://www.postgresql.org/about/policies/archives/

LICENSE

@@ -0,0 +1,19 @@
+Copyright (c) 2020, The 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 POSTGRESQL GLOBAL DEVELOPMENT GROUP HAS BEEN
+ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+THE POSTGRESQL 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 PostgreSQL Global Development Group HAS NO OBLIGATIONS TO
+PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
+