Add README.md

Author: FanwangM

README.md

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+<!--
+This file is part of PyCI.
+
+PyCI 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.
+
+PyCI 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 PyCI. If not, see <http://www.gnu.org/licenses/>.
+-->
+
+[![Python 3](http://img.shields.io/badge/python-3-blue.svg)](https://docs.python.org/3.8/)
+
+# PyCI
+
+PyCI is a flexible quantum chemistry Configuration Interaction library for Python 3.
+
+PyCI is distributed under the GNU General Public License version 3 (GPLv3).
+See http://www.gnu.org/licenses/ for more information.
+
+See http://pyci.qcdevs.org/ for up-to-date installation and usage instructions, as well as for
+tutorials and the API reference.
+
+## Citing PyCI
+
+See the CONTRIBUTORS file.