Merge pull request #3056 from zplovekq/doc_add

[ISSUES #3055]add some doc of the API_Reference_DefaultMQProducer of English.

docs/en/client/java/API_Reference_DefaultMQProducer.md

+## DefaultMQProducer
+---
+### Class introduction
+
+`public class DefaultMQProducer 
+extends ClientConfig 
+implements MQProducer`
+
+>`DefaultMQProducer` is the entry point for an application to post messages, out of the box，ca  quickly create a producer with a no-argument construction. it is mainly responsible for message sending, support synchronous、asynchronous、one-way send. All of these send methods support batch send.  The parameters of the sender can be adjusted through the getter/setter methods , provided by this class. `DefaultMQProducer` has multi send method and each method is slightly different. Make  sure you know the usage before you use it . Blow is a producer example . [see more examples](https://github.com/apache/rocketmq/blob/master/example/src/main/java/org/apache/rocketmq/example/)。
+
+``` java 
+public class Producer {
+    public static void main(String[] args) throws MQClientException {
+        // create a produce with producer_group_name
+        DefaultMQProducer producer = new DefaultMQProducer("ProducerGroupName");
+
+        // start the producer
+        producer.start();
+
+        for (int i = 0; i < 128; i++)
+            try {
+            	// construct the msg
+                Message msg = new Message("TopicTest",
+                        "TagA",
+                        "OrderID188",
+                        "Hello world".getBytes(RemotingHelper.DEFAULT_CHARSET));
+
+                // send sync
+                SendResult sendResult = producer.send(msg);
+
+                // print the result
+                System.out.printf("%s%n", sendResult);
+            } catch (Exception e) {
+                e.printStackTrace();
+            }
+
+        producer.shutdown();
+    }
+}
+```
+
+**Note** : This class is thread safe. It can be safely shared between multiple threads after configuration and startup is complete.
+
+###  Variable 
+
+|Type|Name| description                                                  |
+|------|-------|-------|
+|DefaultMQProducerImpl|defaultMQProducerImpl|The producer's internal default implementation|
+|String|producerGroup|The producer's group|
+|String|createTopicKey| Topics that do not exist on the server are automatically created when the message is sent |
+|int|defaultTopicQueueNums|The default number of queues to create a topic|
+|int|sendMsgTimeout|The timeout for the message to be sent|
+|int|compressMsgBodyOverHowmuch|the threshold of the compress of  message body|
+|int|retryTimesWhenSendFailed|Maximum number of internal attempts to send a message in synchronous mode|
+|int|retryTimesWhenSendAsyncFailed|Maximum number of internal attempts to send a message in asynchronous mode|
+|boolean|retryAnotherBrokerWhenNotStoreOK|Whether to retry another broker if an internal send fails|
+|int|maxMessageSize| Maximum length of message                                    |
+|TraceDispatcher|traceDispatcher| Message trackers. Use rcpHook to track messages              |
+
+### construction method 
+
+|方法名称|方法描述|
+|-------|------------|
+|DefaultMQProducer()| creates a producer with default parameter values             |
+|DefaultMQProducer(final String producerGroup)| creates a producer with producer group name.                 |
+|DefaultMQProducer(final String producerGroup, boolean enableMsgTrace)|creates a producer with producer group name and set whether to enable message tracking|
+|DefaultMQProducer(final String producerGroup, boolean enableMsgTrace, final String customizedTraceTopic)|creates a producer with producer group name and set whether to enable message tracking、the trace topic.|
+|DefaultMQProducer(RPCHook rpcHook)|creates a producer with  a rpc hook.|
+|DefaultMQProducer(final String producerGroup, RPCHook rpcHook)|creates a producer with  a rpc hook and producer group.|
+|DefaultMQProducer(final String producerGroup, RPCHook rpcHook, boolean enableMsgTrace,final String customizedTraceTopic)|all of above.|
+