DISTPG-188 Added Readme and license files

Author: nastena1606

LICENSE

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+Portions Copyright (c) 2021 Percona LLC and/or its affiliates
+
+Portions Copyright (c) 1996-2021, The PostgreSQL Global Development Group
+
+This work is licensed under [the PostgreSQL License]
+(http://www.postgresql.org/about/licence/)	
+
+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 COPYRIGHT HOLDER 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 UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+THE UNIVERSITY OF CALIFORNIA 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 UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.

README.md

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+# Percona Distribution for PostgreSQL documentation.
+
+Welcome to Percona Distribution for PostgreSQL documentation!
+
+## Overview
+
+Percona Distribution for PostgreSQL is a collection of tools to assist you in managing your PostgreSQL database system. It includes the upstream version of PostgreSQL and a selection of extensions that enable solving essential practical tasks efficiently.
+ 
+This repository contains the source files for [Percona Distribution for PostgreSQL documentation](https://www.percona.com/doc/postgresql/13/index.html). The documentation is written in [reStructured text markup language](https://docutils.sourceforge.io/rst.html) and is created using [Sphinx Python Documentation Generator](https://www.sphinx-doc.org/en/master/). 
+
+## Contributing
+
+We welcome all contributions and are always looking for new members that are as dedicated to serving the community as we are. You can reach out to us using our [forums](https://forums.percona.com/c/postgresql/25) and [Jira issue tracker](https://jira.percona.com/projects/DISTPG/issues/DISTPG-16?filter=allopenissues). 
+
+For how to contribute to documentation, read the [Contributing guide](https://github.com/percona/postgresql-docs/blob/13/CONTRIBUTING.md).
+
+## License
+
+Percona Distribution for PostgreSQL documentation is licensed under the [PostgreSQL license](https://opensource.org/licenses/postgresql).