batman-adv: Add dat, mcast, nc and neighbor debugfs files to README

There is a special folder for debugging information:

# ls /sys/kernel/debug/batman_adv/bat0/

# bla_backbone_table  log                 transtable_global

# bla_claim_table     originators         transtable_local

# gateways            socket

# bla_backbone_table  log          neighbors          transtable_local

# bla_claim_table     mcast_flags  originators

# dat_cache           nc           socket

# gateways            nc_nodes     transtable_global

Some of the files contain all sort of status information  regard-

ing  the  mesh  network.  For  example, you can view the table of