Add CC-BY 4.0 badge to README file

Author: 7patricia

README.md

@@ -3,3 +3,8 @@ This repository contains the source files of the Uphold API documentation which
 
 # Contributing
 All submissions are welcome. To submit a change, fork this repo, commit your changes, and send us a pull request.
+
+[![Creative Commons Attribution 4.0 license][license-badge-image]](license-badge-link)
+
+[license-badge-image]: https://i.creativecommons.org/l/by/4.0/80x15.png
+[license-badge-link]: http://creativecommons.org/licenses/by/4.0/