Clarify relationship to nvi

[ant-222]

Please, descibe in README.md the relationship of this nvi2 to the original nvi (site, source) editor: is it a brach regularly sicned with the original, or developed on its own divering way?

[lichray]

Nvi2 is a multibyte fork of the nvi editor for BSD. It is hard to say that https://repo.or.cz/nvi.git is the original nvi, and nvi2 is not branched off it, although contains some of its code. A summary of the differences can be found at https://github.com/lichray/nvi2/wiki/Features