Update to the Governance of data.all

[NickCorbett]

Hi

We are proposing a change to the governance model of data.all (in this PR) which makes the licence implications clearer for contributors. We will remove the statement that says we might ask for a Contributor Licence Agreement (CLA) for some contributions; we won't. Instead, all contributions are made under the terms in LICENCE.txt (ie Apache 2.0).

Thanks

Nick