Update README to use GH migration guide
Update README to use GH migration guide

apascual/braintree_ios,Feverup/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,Feverup/braintree_ios,braintree/braintree_ios,Feverup/braintree_ios,braintree/braintree_ios,Feverup/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,billCTG/braintree_ios,billCTG/braintree_ios,billCTG/braintree_ios,apascual/braintree_ios,apascual/braintree_ios,apascual/braintree_ios