Update README.rst for Django 1.5 style {% url %} use

I spent a while trying to figure out how to get {% url %} to function like {% load url from future %} in Django 1.3/1.4 until I read here about the filter. I think it would be helpful to explicitly mention how to use it with a variable as it took me a bit to figure it out.

Maybe there's a better wording, but something along these lines seems appropriate!