[tor-relays] note to Falback Directory operators: bandwidth wasting bot

[starlight.2018q2 at binnacle.cx]

For the last year a bot of some kind has loaded Fallback
Directory relays with excessive directory document requests.
Some detail can be found in ticket #27921 at
https://trac.torproject.org/projects/tor/ticket/27921.

To determine if your relay is affected, look the relay up in
https://metrics.torproject.org/rs.html, click the 6-Month history
tab, and examine the graph for significantly higher "write bytes
per second" relative to "read bytes per second".

I find this annoying (since I pay for the wasted bandwidth) and
came up with a one-line hack that mitigates the abuse without
impairing the usefulness of the DIR service to normal relays
and clients.  If you compile your daemon from source and are
interested contact me directly for the patch.