Deprecate this project

[juanjoDiaz]

Once https://github.com/zemirco/json2csv/pull/235 is merged and released, this project will be obsolete.

I'd advise that you also:

• Add [Deprecated] to the title or description of the repo.
• Deprecate it on npm using https://docs.npmjs.com/cli/deprecate

[juanjoDiaz]

json2csv v4.X is out, so we are good to deprecate this project.