Updated README with install instructions (#11)

Author: idk3

README.rst

@@ -3,6 +3,21 @@ FermiLib plugin to interface with Psi4
 
 This repository contains a plugin which allows `FermiLib <http://github.com/ProjectQ-Framework/FermiLib>`__ (licensed under Apache 2) to interface with the open-source quantum chemistry package `Psi4 <http://www.psicode.org>`__ (licensed under GNU Lesser General Public License version 3). 
 
+Installation
+------------
+
+To install this plugin, clone this git repo and then change directory to the top level folder. Then, run
+
+.. code-block:: bash
+
+  python -m pip install -e .
+
+Alternatively, one can install using pip with the command
+
+.. code-block:: bash
+
+  python -m pip install fermilibpluginpsi4
+
 License
 -------
 Copyright (C) 2017 FermiLib Developers