[tor-relays] ExtORPort notice

[Delton Barnes]

Upon upgrading obfsproxy to 0.2.6 and Tor to 0.2.5.1-alpha-dev
(git-f63b394d90583b77+96972c4) for scramblesuit, I got this in the Tor log:

Feb 15 04:40:03.000 [notice] We are a bridge with a pluggable transport
proxy but the Extended ORPort is disabled. The Extended ORPort helps Tor
communicate with the pluggable transport proxy. Please enable it using
the ExtORPort torrc option.

How should this be set?  What does it do?  I saw some web pages
suggesting "ExtORPort 6699" for statistics-gathering purposes.

Thanks,
Delton