Add a note about associations to the README
Add a note about associations to the README

peterjm/activeresource,eileencodes/activeresource,rjordan/activeresource,1776/activeresource,chadzilla2080/activeresource,rails/activeresource,SweeD/activeresource,Shopify/activeresource,repinel/activeresource,yui-knk/activeresource,maclover7/activeresource,delameko/activeresource,wyattisimo/activeresource,om-nishu-trantor/activeresource