Staging: batman-adv: Update the README for mainline usage.

The module now only compiles inside mainline, so remove the comments
about different versions of the kernel it can be used with. Also
update comments about building with debug enabled and how to use the
VIS data now that it no longer natively outputs dot or JSON.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>