add README file

POWER/README

+Python Open Source Waveform ExtractoR: An open source, python package to monitor and post-process numerical relativity simulations
+Author(s)    : Daniel Johnson <dsjohns2@illinois.edu>
+               E. A. Huerta <elihu@illinois.edu>
+               Roland Haas <rhaas@ncsa.illinois.edu>
+Copyright    : (c) 2017 The Board of Trustees of the University of Illinois
+Maintainer(s): Daniel Johnson, E. A. Huerta, Roland Haas
+License      : NCSA
+
+1. Purpose
+
+POWER is an open source, python package to monitor the status and progress of
+numerical relativity simulations, and to post-process the data products of
+these simulations to compute the gravitational wave strain at future null
+infinity.