Add open source attribution.
Add open source attribution.

mapleoin/braintree_python,braintree/braintree_python,felixonmars/braintree_python,mapleoin/braintree_python,felixonmars/braintree_python,felixonmars/braintree_python,mapleoin/braintree_python,braintree/braintree_python,braintree/braintree_python