mention freeness in README

README

-Kwant is a Python package for numerical quantum transport calculations. It
-exposes the natural concepts of the theory of quantum transport (lattices,
-symmetries, electrodes, orbital/spin/electron-hole degrees of freedom) in a
-simple and transparent way. Kwant offers direct support for calculations of
-transport properties (conductance, noise, scattering matrix), dispersion
-relations, modes, wave functions, various Green’s functions, and
+Kwant is a free (open source) Python package for numerical quantum transport
+calculations. It exposes the natural concepts of the theory of quantum transport
+(lattices, symmetries, electrodes, orbital/spin/electron-hole degrees of
+freedom) in a simple and transparent way. Kwant offers direct support for
+calculations of transport properties (conductance, noise, scattering matrix),
+dispersion relations, modes, wave functions, various Green’s functions, and
 out-of-equilibrium local quantities. Other computations involving tight-binding
 Hamiltonians can be implemented easily.