Info regarding what matplotlib.use command to use should be added to README

After cloning the repo onto Linux, it took me a good few hours to discover that to get the python program to work, I had to comment matplotlib.use('MacOSX') and uncomment matplotlib.use('GTKCairo') from sim_1.py.

The error was "cannot import name _macosx'.

If a sentence or two was added to the readme explaining this, it would make things clearer for Linux neewbies like me :-)
