Update README.md to mention that pacing is required for BBR (e.g., on Linux use fq instead of fq_codel)

It is mentioned in #1180 but could be stated explicitly in my opinion. Also see #1513.

Or maybe add a note to the argument parser but this seems wrong because it's not limited to that specific app.

Checking if SO_TXTIME is provided by the socket would be nice but I don't know if this is possible.