UNLICENSED is used for non-open source projects. Since README.md mentions that a project is free for anyone to use, I suggest changing the licence to MIT to make it true open source.
If you do not want to specify a license or if the source code is not open-source, please use the special value UNLICENSED. Note that UNLICENSED (no usage allowed, not present in SPDX license list) is different from UNLICENSE (grants all rights to everyone).
UNLICENSED is used for non-open source projects. Since README.md mentions that a project is free for anyone to use, I suggest changing the licence to MIT to make it true open source.
Source: https://docs.soliditylang.org/en/latest/layout-of-source-files.html