Move license to LICENSE.txt in root directory

[danielklecha]

License is available in \LICENSES\BSD-3-Clause.txt.

1. GitHub doesn't display your license in about section.
2. It's impossible to get this license using automated tools like DotnetThirdPartyNotices.

Could you rename license file and move it to \LICENSE.txt?

[AaronAtDuo]

@danielklecha We use some internal tooling that relies on the licenses being under a LICENCES directory, but I'll see what I can do.

Just out of curiosity, how does that tool deal with repos that have multiple licenses?