contrib/diff-highlight: mention interactive.diffFilter

contrib/diff-highlight: mention interactive.diffFilter

When the README for diff-highlight was written, there was no way to
trigger it for the `add -p` interactive patch mode. We've since grown a
feature to support that, but it was documented only on the Git side.
Let's also let people coming the other direction, from diff-highlight,
know that it's an option.

Suggested-by: Isaac Oscar Gariano <IsaacOscar@live.com.au>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>