Be more explicit about license

To avoid misunderstandings like in #11 (closed), I propose to amend the text in README.md#License.

I was thinking something like this:

By submitting any contribution to this repository (such as a new translation, or an improvement to a translation), you accept to release it under the license Creative Commons Attribution 4.0 International (CC By 4.0). You acknowledge that you understand this license, and in particular that you cannot revoke the permissions you give.

You are of course still free to make derivatives and translations of Pepper&Carrot and distribute them under other licenses, so long as you comply with CC By 4.0. But if you want to have them included in this repo and on the official website, you have to release them under CC By 4.0.

This does not apply to content in the fonts/ directory.

Let's collaborate on this text at https://mensuel.framapad.org/p/pepper-carrot-license-grant-AOh3LljY6s.