Mentioning Snowflake in the TorRelayGuide

Please mention and link to the guide on running a Snowflake bridge:

• https://trac.torproject.org/projects/tor/wiki/doc/Snowflake#HowtorunaSnowflakeproxy

In particular, as an alternative to running a relay at home, or as an additional alternative to the addition here https://trac.torproject.org/projects/tor/wiki/TorRelayGuide?action=diff&version=202