fanout group description suggestion

[simonhf]

I was trying to learn how fanout groups work and didn't realize until very late that nearly all fanout group applications and examples online just use a single fanout group!

When I was reading your definition I got confused that the number of fanout groups might be the same as the N threads:

"A related feature, fanout groups, exists so that you can capture from N processes or threads at the same time, and each thread will see 1/Nth of the traffic."

This definition in README.md would have helped me, and maybe would help somebody else:

"A related feature, fanout groups, exists so that X (usually only 1) fanout group(s) of Y processes or threads can capture at the same time; each thread in a group will see 1/Yth of the traffic; each group sees its own duplicate of the traffic."