add user guide docs for kubectl exec, proxy, and port-forward

docs/user-guide/getting-into-containers.md

@@ -0,0 +1,49 @@
+#Getting into containers: kubectl exec
+Developers can use `kubectl exec` to run commands in a container. This guide demonstrates two use cases.
+
+##Using kubectl exec to check the environment variables of a container
+Kubernetes exposes [services](../../docs/services.md#environment-variables) through environment variables. It is convenient to check these environment variables using `kubectl exec`.
+
+
+We first create a pod and a service,
+```
+$ kubectl create -f examples/guestbook/redis-master-controller.yaml
+$ kubectl create -f examples/guestbook/redis-master-service.yaml
+```
+wait until the pod is Running and Ready,
+```
+$ kubectl get pod
+NAME                 READY     REASON       RESTARTS   AGE
+redis-master-ft9ex   1/1       Running      0          12s
+```
+then we can check the environment variables of the pod, 
+```
+$ kubectl exec redis-master-ft9ex env
+...
+REDIS_MASTER_SERVICE_PORT=6379
+REDIS_MASTER_SERVICE_HOST=10.0.0.219
+...
+```
+We can use these environment variables in applications to find the service.
+
+
+## Using kubectl exec to check the mounted volumes
+It is convenient to use `kubectl exec` to check if the volumes are mounted as expected.
+We first create a Pod with a volume mounted at /data/redis,
+```
+kubectl create -f examples/walkthrough/pod2.yaml
+```
+wait until the pod is Running and Ready,
+```
+$ kubectl get pods
+NAME      READY     REASON    RESTARTS   AGE
+storage   1/1       Running   0          1m
+```
+we then use `kubectl exec` to verify that the volume is mounted at /data/redis,
+```
+$ kubectl exec storage ls /data
+redis
+```
+
+
+[![Analytics](https://kubernetes-site.appspot.com/UA-36037335-10/GitHub/docs/user-guide/getting-into-containers.md?pixel)]()