Add instruction to enable onion service v3

[mrphs]

Onion Services version 3 provide better crypto and have improved security features
More info: https://trac.torproject.org/projects/tor/wiki/doc/NextGenOnions

Related: #23

[kargig]

I'm not against suggesting v3 onion services at some point, but I think they're still less stable than v2 ones.

What do others think ?

[mrphs]

I just went over the open tickets and have to say I'm not aware of any instability and no bugs that could affect this project. Do you have any specific issue in mind? I've been using onion v3 by default for a while and haven't noticed any weirdness and considering the security improvements, I think it would be beneficial for everybody to switch to v3. That being said, I'm not even suggesting to make v3 the default. You can have both legacy and v3 onions at the same time and if the other end is using an older version of Tor (yikes, I hope no one is running such old version of Tor), it can fall back to v2.

[asn-d6]

I concur with mrphs. IMO, hsv3 is stable enough to be used by projects. There is currently no known hsv3 bug that could impact reachability or stability.

If you find such a bug, please let us know and we will fix it ASAP.