add user guidance of env plugin

Signed-off-by: Thor-wl <13164644535@163.com>

docs/user-guide/how_to_use_env_plugin.md

@@ -0,0 +1,82 @@
+# Env Plugin User Guidance
+
+## Background
+**Env Plugin** is designed for business that a pod should be aware of its index in the task such as [MPI](https://www.open-mpi.org/)
+and [TensorFlow](https://tensorflow.google.cn/). The indexes will be registered as **environment variables** automatically
+when the Volcano job is created. For example, a tensorflow job consists of *1* ps and *2* workers. And each worker maps 
+to a slice of raw data. In order to make the workers be aware of its target slice, they get their index in the environment
+variables.
+
+## Key Points
+* The index keys of the environment variables are `VK_TASK_INDEX` and `VC_TASK_INDEX`, they have the same value.
+* The value of the indexes is a number which ranges from `0` to `length - 1`. The `length` equals to the number of replicas 
+of the task. It is also the index of the pod in the task. 
+
+## Examples
+```yaml
+apiVersion: batch.volcano.sh/v1alpha1
+kind: Job
+metadata:
+  name: tensorflow-dist-mnist
+spec:
+  minAvailable: 3
+  schedulerName: volcano
+  plugins:
+    env: []   ## Env plugin register, note that no values are needed in the array.
+    svc: []
+  policies:
+    - event: PodEvicted
+      action: RestartJob
+  queue: default
+  tasks:
+    - replicas: 1
+      name: ps
+      template:
+        spec:
+          containers:
+            - command:
+                - sh
+                - -c
+                - |
+                  PS_HOST=`cat /etc/volcano/ps.host | sed 's/$/&:2222/g' | sed 's/^/"/;s/$/"/' | tr "\n" ","`;
+                  WORKER_HOST=`cat /etc/volcano/worker.host | sed 's/$/&:2222/g' | sed 's/^/"/;s/$/"/' | tr "\n" ","`;
+                  export TF_CONFIG={\"cluster\":{\"ps\":[${PS_HOST}],\"worker\":[${WORKER_HOST}]},\"task\":{\"type\":\"ps\",\"index\":${VK_TASK_INDEX}},\"environment\":\"cloud\"};   ## Get the index from the environment variable and configure it in the TF job.
+                  python /var/tf_dist_mnist/dist_mnist.py
+              image: volcanosh/dist-mnist-tf-example:0.0.1
+              name: tensorflow
+              ports:
+                - containerPort: 2222
+                  name: tfjob-port
+              resources: {}
+          restartPolicy: Never
+    - replicas: 2
+      name: worker
+      policies:
+        - event: TaskCompleted
+          action: CompleteJob
+      template:
+        spec:
+          containers:
+            - command:
+                - sh
+                - -c
+                - |
+                  PS_HOST=`cat /etc/volcano/ps.host | sed 's/$/&:2222/g' | sed 's/^/"/;s/$/"/' | tr "\n" ","`;
+                  WORKER_HOST=`cat /etc/volcano/worker.host | sed 's/$/&:2222/g' | sed 's/^/"/;s/$/"/' | tr "\n" ","`;
+                  export TF_CONFIG={\"cluster\":{\"ps\":[${PS_HOST}],\"worker\":[${WORKER_HOST}]},\"task\":{\"type\":\"worker\",\"index\":${VK_TASK_INDEX}},\"environment\":\"cloud\"};
+                  python /var/tf_dist_mnist/dist_mnist.py
+              image: volcanosh/dist-mnist-tf-example:0.0.1
+              name: tensorflow
+              ports:
+                - containerPort: 2222
+                  name: tfjob-port
+              resources: {}
+          restartPolicy: Never
+```
+When config env plugin in the tensorflow job above, all the pods will have 2 environment variables `VK_TASK_INDEX` and 
+`VC_TASK_INDEX`. Considering the 2 workers, you will get that their names are `tensorflow-dist-mnist-worker-0` and
+`tensorflow-dist-mnist-worker-1`. And the corresponding values of the index environment variables are `0` and `1`.
+## Note
+* Because of historical reasons, environment variables `VK_TASK_INDEX` and `VC_TASK_INDEX` both exist, `VK_TASK_INDEX` will
+be **deprecated** in the future releases.
+* No value are needed when register env plugin in the volcano job.