Add a Membership to the project and returns a
        :class:`Membership` resource.

        :param email: email for :class:`Membership`
        :param role: role for :class:`Membership`
        :param attrs: role for :class:`Membership`
        :param attrs: optional :class:`Membership` attributes

def add_membership(self, email, role, **attrs):
        """
        Add a Membership to the project and returns a
        :class:`Membership` resource.

        :param email: email for :class:`Membership`
        :param role: role for :class:`Membership`
        :param attrs: role for :class:`Membership`
        :param attrs: optional :class:`Membership` attributes
        """
        return Memberships(self.requester).create(
            self.id, email, role, **attrs
        )