Add NEWS.unknown file to correct versions of introduction

Add NEWS.unknown file to correct versions of introduction

It is not intended for the human reader, but to give less wrong versions
of first appearance displayed by `describe-function` or
`describe-variable` that would otherwise be derived from a notice in a
more recent NEWS* file.

It is better to add items to this file than changing historical NEWS*
files when the actual version of introduction is not known.

* etc/NEWS.unknown: New file.  Add entry for `nthcdr`.