Update repo documentation: contributing, copying and updated readme

Author: Alexandru Cheltuitor

CONTRIBUTING.md

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+# Contribution Policy
+
+By making a contribution to this project:
+
+1.    I assign any and all copyright related to the contribution to Proton Technologies AG;
+2.    I certify that the contribution was created in whole by me;
+3.    I understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information I submit with it) is maintained indefinitely and may be redistributed with this project or the open source license(s) involved.

COPYING.md

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+## Copying
+
+Copyright (c) 2021 Proton Technologies AG
+
+Python Proton Client is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+Python Proton Client is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with ProtonVPN. If not, see [https://www.gnu.org/licenses](https://www.gnu.org/licenses/).

README.md

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-Proton API Python Client
-=============
+# Proton API Python Client
+
+Copyright (c) 2021 Proton Technologies AG
+
+This repository holds the Proton Python Client.
+For licensing information see [COPYING](COPYING.md).
+For contribution policy see [CONTRIBUTING](CONTRIBUTING.md).
+
+## Description
+The Python Proton Client is intended for every Proton service user.
+
+You can download the latest stable release, either from our official repositories or directly on the [official GitHub repository](https://github.com/ProtonMail/proton-python-client/releases/latest).
 
 ## Dependencies
 | **Python** | **Debian** | **Fedora** | **Arch** |