Staging: batman-adv: Update README about vis raw output

We are now having a newer, more neutral vis output so that we won't have
to change the kernelmodule for adding support of new vis output formats.
This patch adds an explanation about this in the README file of
batman-adv and removes the description about the dot/json format (they
will be added to the README of batctl).

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>