Add CC-BY-SA License.

[Mariatta]

Text copied from https://choosealicense.com/licenses/cc-by-sa-4.0/

Closes #22

[vstinner]

Can you please also put the same change at the bottom of index.rst?

[vstinner]

As I wrote in the issue #22, I suggest to distribute code examples as CC0.

CC BY-SA for code examples is not convenient :-(

Or maybe we need a single license for everything without the annoying "BY" part of CC BY-SA.

[Mariatta]

Thanks. I added a sentence saying that code examples are CC0, but everything else is CC BY-SA.
Will this work?
I kept the main LICENSE file in the repo as CC BY-SA.

[vstinner]

I requested the review of the most active contributors to the documentation, since it's a license change.

[vstinner]

LGTM. But it would be nice to wait the feedback from the other contributors to this documentation, since it's a license change.

I'm not sure that it's useful to have a copy of the license in the LICENSE file. If you think that it's useful, would you mind to also add the CC0 license?

[vxgmichel]

I'm ok with it!

[Mariatta]

Ok. I added a new file: LICENSE_CC0 which contains the text for CC0 License. :)

[matrixise]

Hi @vstinner @Mariatta @hoh and @vxgmichel

I am going to merge this one if you agree.

[ludovic-gasc]

LGTM