Missing documentation on benchmark tool in README.md

[aseigo]

The README.md contains this:

"### C Receiver / erlang-czmq Sender

This test measures the throughput of using an erlang-czmq sender with a C
receiver.

Start the receiver as with the C / C test above.

Next,"

Next ... what? :) It would be nice to know how the benchmark tool is intended to work without reading through the code itself.