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. 
Merge pull request #50 from mrooney/patch-2

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