add qsymm to extensions

content/extensions.rst

@@ -36,3 +36,11 @@ following two papers:
 
 + `Towards realistic time-resolved simulations of quantum devices <https://dx.doi.org/10.1007/s10825-016-0855-9>`_
 + `Numerical simulations of time resolved quantum electronics <https://dx.doi.org/10.1016/j.physrep.2013.09.001>`_
+
+Qsymm: Symmetry finder and symmetric Hamiltonian generator
+----------------------------------------------------------
+**Get the code**: https://gitlab.kwant-project.org/qt/qsymm
+
+A package that makes symmetry analysis simple. It automatically generates model Hamiltonians from symmetry constraints and finds the full symmetry group of your Hamiltonian. Works with tight-binging and k dot p Hamiltonians and supports all kinds of symmetries, including conserved quantities, space group symmetries, time reversal, particle-hole and all combinations of these.
+
++ `Qsymm: algorithmic symmetry finding and symmetric Hamiltonian generation <http://dx.doi.org/10.1088/1367-2630/aadf67>`_