README.md: add a quick description about xmltv

Make easier for users to work with xmltv files by adding a
quick comment at the Kaffeine's readme file.

Also, add a small script with an example about how to map
XMLTV channels into DVB ones.

BUG: 405278

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>