Added SNI Routing tutorial

Author: vfarcic

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.8 AS build
+FROM golang:1.9 AS build
 ADD . /src
 WORKDIR /src
 RUN go get -d -v -t

mkdocs.yml

@@ -8,6 +8,7 @@ pages:
     - Managing SSL Certificates: certs.md
     - Blocking Requests: blocking.md
     - Debugging: debugging.md
+    - Using SNI Routing: sni-routing.md
     - Configuring Non-Swarm Services: non-swarm.md
   - Configuration: config.md
   - Usage: usage.md