Update README and NEWS for ports directory removal.

Update README and NEWS for ports directory removal.

This patch updates README to remove a mention of the ports directory.
It also adds a NEWS item for the merge of ports into the main sysdeps
tree (I think it's NEWS-worthy, although not strictly a user-visible
feature).

Other remaining ports references to resolve: a comment in
manual/signal.texi (not giving a literal path, but maybe should change
anyway); a comment in config.h.in (path should be updated);
scripts/list-sources.sh (appears to date back to ports being a
separate repository).

	* README: Do not mention ports directory.