When submitting a pull request for the first time, you will need to agree to the contributor license agreement (for individuals or entities). To do this, in the pull request please create a file with a name like /contributors/{github_username}.md, and in the content of that file indicate your agreement. An example of what that file should contain can be seen in adam-p's agreement file.
(This method of CLA "signing" is borrowed from Medium's open source project.)