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>

contrib/diff-highlight/README

@@ -58,6 +58,14 @@ following in your git configuration:
 	diff = diff-highlight | less
 ---------------------------------------------
 
+If you use the interactive patch mode of `git add -p`, `git checkout
+-p`, etc, you may also want to configure it to be used there:
+
+---------------------------------------------
+[interactive]
+        diffFilter = diff-highlight
+---------------------------------------------
+
 
 Color Config
 ------------