add numpy as runtime req (#1308)

doc/install/build_install.rst

@@ -120,6 +120,7 @@ Python
 ~~~~~~
 
 Arbor has a Python frontend, for which a minimum of Python 3.6 is required.
+In addition, `numpy` is a runtime requirement for the Python package.
 In order to use MPI in combination with the python frontend the
 `mpi4py <https://mpi4py.readthedocs.io/en/stable/install.html#>`_
 Python package is recommended. See :ref:`install-python` for more information.