Amend greeting message with pointer to weather.torproject.org

In log_new_relay_greeting we have a welcome message pointing to our lifecycle document:

  tor_log(LOG_NOTICE, LD_GENERAL, "You are running a new relay. "
         "Thanks for helping the Tor network! If you wish to know "
         "what will happen in the upcoming weeks regarding its usage, "
         "have a look at https://blog.torproject.org/lifecycle-of-a"
         "-new-relay");

We should amend that message and pointing to our new Tor Weather service at weather.tpo as well offering operators an easy way to get notified e.g. in case their relay goes down.