Update README.md

Description of the library

README.md

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-PowerAPI
+Overview
 ========
+PowerAPI is a middleware toolkit for buidling software-defined power meters. Software-defined power meters are software librairies that estimates the power consumption of software in real-time. PowerAPI supports the acquisition of raw metrics from a wide diversity of sensors (physical meters, processor interfaces, hardware counters, OS counters) and the delivery of power consumptions via different interfaces (filesystem, network, web, graphical).
 
-Implementation of the PowerAPI library
+
+Licence
+=======
+Copyright (C) 2011-2014 Inria, University Lille 1.
+
+PowerAPI is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
+
+PowerAPI 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 Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License along with PowerAPI. If not, please consult http://www.gnu.org/licenses/agpl-3.0.html.