Mention the Open Source License in the README and/or documentation.

[Ly0n]

Describe the feature you'd like to see

Hi PyPSA-Earth community,
If you read the README and/or the documentation, you might think that PyPSA-Earth is not an Open Source project, because the LICENSE is not automatically detected by GitHub. Also, there is no licence mentioned in the README. This might lead some people to think that the project is just 'open code' and not open source when looking on the documentation or README. So I would recommend adding a small note in the README in the form of a badge or a small section linking to the licence. Ideally, a LICENSE file should be automatically recognised by GitHub (this is the case when the LICENSE logo appears on the right-hand side), but this is a problem if you are using REUSE and multiple licenses. Many GitHub user have already complained about this. See the last comments here: todogroup/gh-issues#72

Maybe a small note to the README might help here like this:

``
This repository uses multiple licenses:

1. LICENSES/AGPL-3.0-or-later
   See LICENSES/AGPL-3.0-or-later.txt for details.

2. CC-BY-4.0 License
   See LICENSES/CC-BY-4.0 for details.

3. CC0-1.0.txt
   See LICENSES/CC0-1.0.txt for details.

Refer to the individual files in the LICENSES directory for the full license texts.
``

[ekatef]

Hey @Ly0n, completely agree and thanks a lot for pointing this out!

Apologies for a delay with the response: I just have hoped to answer suggesting a fix but it has not yet been feasible. Adding tags to the issue to keep it in sight

[ekatef]

@GbotemiB may it be a good idea to add this work to the plan on documentation improvements?