README: Suggest running `python' instead of shebang
README: Suggest running `python' instead of shebang

Because the shebang in manage.py points to the system python instead of
the python in the virtualenv, suggest executing manage.py with `python
manage.py' instead of `./manage.py'. I don't think it makes a
difference usually, but it's safer I think.

global-humanitarians-unite/ghu,global-humanitarians-unite/ghu,global-humanitarians-unite/ghu,global-humanitarians-unite/ghu