d: add rpmdiff.1

Add a basic manpage for rpmdiff.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
d/changelog: add rpmdiff notice

The rpmdiff file has different copyright (GPL-2 only vs GPL-2+).

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>