Add README Xcode 12 warning note (#567)
Add README Xcode 12 warning note (#567)

Co-authored-by: sestevens <40e0c00ea33d9a847173fa91d68d8fb0da6ba4d6@getbraintree.com>
braintree/braintree_ios,braintree/braintree_ios,braintree/braintree_ios,braintree/braintree_ios