[siconos] add license tag in readme
| **`ubuntu-14-04-gcc-atlas-lapack`** |
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|[![Build Status](https://travis-ci.org/siconos/siconos.svg?branch=master)](https://travis-ci.org/siconos/siconos)|
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-|[![Version](https://img.shields.io/github/release/siconos/siconos.svg)](https://github.com/siconos/siconos/releases/latest)|
+|[![Version](https://img.shields.io/github/release/siconos/siconos.svg)](https://github.com/siconos/siconos/releases/latest)
+ [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/siconos/siconos/blob/master/COPYING)|
A software package for the modeling and simulation of nonsmooth dynamical systems in C++ and in Python.
