Add a LICENSE file with the plain text of CC-BY-4.0

[vaitkus]

This PR licenses all files in the repository under CC-BY-4.0 as discussed in issue COMCIFS/cif_core#247.

[vaitkus]

Before merging this PR it should be clarified if all files in the repository can be licensed under CC-BY-4.0. Specifically, the various draft versions of the topoCIF chapter for ITC volume G and the files under the java directory may already be placed under some implicit license.

@jamesrhester, @nautolycus any ideas on this?

[jamesrhester]

Our decision on licensing related only to the dictionary files, so I don't think we can license all files in a repository if there are non-dictionary files present as in this case.