Handling of license files outside of the LICENSE folder

[cordlandwehr]

In the Yocto packaging world there I have the following use case:

• I have a Yocto meta-layer that is in an abstract view a git repository with build description files
• Yocto layers can provide license text files, which in later build steps need to be included in builds of the respective software packages
• for Yocto purposes, those files have no ".txt" extension and are located in some other folder than LICENSES/ for technical reasons that are not fast/simple to change
• most of those license file do not provide license texts for files inside the repository but for files in external repositories

All this makes reuse lint fail for me unless I assign a license to the license text file. By the same arguments why all files in LICENSES/ are also not included in the linter, I propose to add an option to e.g. the REUSE.toml file to define additional folders of licenses for which reuse lint does not complain when their licenses are not additionally stated.