Document etcd requirement and fix segmentation fault

[LorenzBischof]

trafficstars

I see no reference to etcd in the README and if I comment it out in the sample-conf.yaml I receive a segmentation fault.

[wpaulino]

It is not optional at the moment, it is used to store the LSAT macaroon root keys and Tor onion service private keys (if enabled). Things should be fairly abstracted however to allow for a different database to be used.

[LorenzBischof]

I changed the issue title. If I have time, I will see if I can create a pull request in the next few days.

[guggero]

That would be nice, thanks!