Added init container documentation.

Based loosely on the design proposal.

docs/user-guide/petset/bootstrapping/index.md

@@ -88,7 +88,7 @@ vm-1 # printf "GET / HTTP/1.0\r\n\r\n" | netcat vm-0.ub 80
 It's worth exploring what just happened. Init containers run sequentially *before* the application container. In this example we used the init container to copy shared libraries from the rootfs, while preserving user installed packages across container restart.
 
 ```yaml
-pod.alpha.kubernetes.io/init-containers: '[
+pod.beta.kubernetes.io/init-containers: '[
     {
         "name": "rootfs",
         "image": "ubuntu:15.10",

docs/user-guide/petset/bootstrapping/petset_peers.yaml

@@ -29,7 +29,7 @@ spec:
         app: nginx
       annotations:
         pod.alpha.kubernetes.io/initialized: "true"
-        pod.alpha.kubernetes.io/init-containers: '[
+        pod.beta.kubernetes.io/init-containers: '[
             {
                 "name": "peerfinder",
                 "image": "gcr.io/google_containers/peer-finder:0.1",