README: IAM roles link changed to point to IAM user guide
README: IAM roles link changed to point to IAM user guide

mixpanel-platform/chef-handler-sns,zuazo/chef-handler-sns,mevic/chef-handler-sns,onddo/chef-handler-sns,mixpanel-platform/chef-handler-sns,easybiblabs/chef-handler-sns,mevic/chef-handler-sns,zuazo/chef-handler-sns,onddo/chef-handler-sns