Expand guidance in README about licensing

[larsvilhuber]

template_README/template-README.md

Line 53 in 42894e3

> INSTRUCTIONS: Most data repositories provide for a default license, but do not impose a specific license. Authors should actively select a license. This should be provided in a LICENSE.txt file, separately from the README, possibly combined with the license for any code. Some data may be subject to inherited license requirements, i.e., the data provider may allow for redistribution only if the data is licensed under specific rules - authors should check with their data providers. For instance, a data use license might require that users - the current author, but also any subsequent users - cite the data provider. Licensing can be complex. Some non-legal guidance may be found [here](https://social-science-data-editors.github.io/guidance/Licensing_guidance.html). For multiple licenses within a data package, the `LICENSE.txt` file might contain the concatenation of all the licenses that apply (for instance, a custom license for one file, plus a CC-BY license for another file).

[larsvilhuber]

Also clarify that the LICENSE.txt needs to be created by the author, and adapted. Link to guidance.

template_README/template-README.md

Line 231 in 42894e3

The code is licensed under a MIT/BSD/GPL [choose one!] license. See [LICENSE.txt](LICENSE.txt) for details.