[RIP-9] Add the introduction of the batch message samples in RocketMQ (#792)

[RIP-9] Add the introduction of the batch message samples in RocketMQ

docs/en/Example_Batch.md

+# Batch Message Sample
+------
+Sending messages in batch improves performance of delivering small messages. Messages of the same batch should have: same topic, same waitStoreMsgOK and no schedule support. You can send messages up to 4MiB at a time, but if you need to send a larger message, it is recommended to divide the larger messages into multiple small messages of no more than 1MiB.
+### 1 Send Batch Messages
+If you just send messages of no more than 4MiB at a time, it is easy to use batch:
+```java
+String topic = "BatchTest";
+List<Message> messages = new ArrayList<>();
+messages.add(new Message(topic, "TagA", "OrderID001", "Hello world 0".getBytes()));
+messages.add(new Message(topic, "TagA", "OrderID002", "Hello world 1".getBytes()));
+messages.add(new Message(topic, "TagA", "OrderID003", "Hello world 2".getBytes()));
+try {
+    producer.send(messages);
+} catch (Exception e) {
+    e.printStackTrace();
+    //handle the error
+}
+```
+### 2 Split into Lists
+The complexity only grow when you send large batch and you may not sure if it exceeds the size limit (4MiB). At this time, you’d better split the lists:
+```java
+public class ListSplitter implements Iterator<List<Message>> {
+    private final int SIZE_LIMIT = 1000 * 1000;
+    private final List<Message> messages;
+    private int currIndex;
+    public ListSplitter(List<Message> messages) {
+            this.messages = messages;
+    }
+    @Override public boolean hasNext() {
+        return currIndex < messages.size();
+    }
+    @Override public List<Message> next() {
+        int nextIndex = currIndex;
+        int totalSize = 0;
+        for (; nextIndex < messages.size(); nextIndex++) {
+            Message message = messages.get(nextIndex);
+            int tmpSize = message.getTopic().length() + message.getBody().length;
+            Map<String, String> properties = message.getProperties();
+            for (Map.Entry<String, String> entry : properties.entrySet()) {
+                tmpSize += entry.getKey().length() + entry.getValue().length();
+            }
+            tmpSize = tmpSize + 20; //for log overhead
+            if (tmpSize > SIZE_LIMIT) {
+                //it is unexpected that single message exceeds the SIZE_LIMIT
+                //here just let it go, otherwise it will block the splitting process
+                if (nextIndex - currIndex == 0) {
+                   //if the next sublist has no element, add this one and then break, otherwise just break
+                   nextIndex++;  
+                }
+                break;
+            }
+            if (tmpSize + totalSize > SIZE_LIMIT) {
+                break;
+            } else {
+                totalSize += tmpSize;
+            }
+    
+        }
+        List<Message> subList = messages.subList(currIndex, nextIndex);
+        currIndex = nextIndex;
+        return subList;
+    }
+}
+
+// then you could split the large list into small ones:
+ListSplitter splitter = new ListSplitter(messages);
+while (splitter.hasNext()) {
+   try {
+       List<Message>  listItem = splitter.next();
+       producer.send(listItem);
+   } catch (Exception e) {
+       e.printStackTrace();
+       // handle the error
+   }
+}
+```
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