adds brief description

README.md

 # Solarmod-code
 
 Semi-analytical model for solar modulation in two dimensions.
+
+A minimal test script is shown in test.py in python and test.cpp in c++.  In
+both cases the local interstellar spectrum is loaded from a file, the spectrum
+is modulated using the parametrisation presented in arXiv:1909.01154 and given
+scale factors g1 and g2 and then write the modulated spectrum to a file.