Add TLS suggestions to documentation

[hltbra]

I don't have any plans to implement native TLS to dredis, but I think it's a good idea to suggest stunnel in front of dredis if you want to in-transit encryption.

At the end of chapter 7 (Security Techniques) of Redis Essentials, there's a short stunnel + Redis HOWTO.
More details can be found at http://bencane.com/2014/02/18/sending-redis-traffic-through-an-ssl-tunnel-with-stunnel/