[docs] Add a note about setting nf_conntrack_acct

[oofnikj]

Not sure if this is relevant outside of my niche use case, but I spent several hours trying to debug why all my packet counters were zero and finally realized that I needed the following sysctl:

net.netfilter.nf_conntrack_acct = 1

This was set to 0 on my system. Setting it to 1 and restarting nlbwmon fixed it.
Might be helpful for someone in the future.