Add "update fallbacks" to "Preliminaries" in Tor's doc/HACKING/ReleasingTor.md
When we added fallback directory mirrors, we should have also listed them as something we need to update every major release:
- Early in the alpha series for each new major release, at least a month before the code freeze, update the list of fallback directory mirrors using the instructions in: https://trac.torproject.org/projects/tor/wiki/doc/UpdatingFallbackDirectoryMirrors