[RIP-9] Add doc of best practice for broker (#774)

[RIP-9] Add doc of best practice for broker 

docs/en/Operations_Broker.md

+# 3 Broker
+
+## 3.1 Broker Role
+Broker Role is ASYNC_MASTER, SYNC_MASTER or SLAVE. If you cannot tolerate message missing, we suggest you deploy SYNC_MASTER and attach a SLAVE to it. If you feel ok about missing, but you want the Broker to be always available, you may deploy ASYNC_MASTER with SLAVE. If you just want to make it easy, you may only need a ASYNC_MASTER without SLAVE.
+## 3.2 FlushDiskType
+ASYNC_FLUSH is recommended, for SYNC_FLUSH is expensive and will cause too much performance loss. If you want reliability, we recommend you use SYNC_MASTER with SLAVE.
+## 3.3 Broker Configuration
+| Parameter name                           | Default                        | Description                                                         |
+| -------------------------------- | ----------------------------- | ------------------------------------------------------------ |
+| listenPort                    | 10911              | listen port for client |
+| namesrvAddr       | null                         | name server address     |
+| brokerIP1 | InetAddress for network interface                         | Should be configured if having multiple addresses |
+| brokerIP2 | InetAddress for network interface                         | If configured for the Master broker in the Master/Slave cluster, slave broker will connect to this port for data synchronization   |
+| brokerName        | null                         | broker name                           |
+| brokerClusterName                     | DefaultCluster                  | this broker belongs to which cluster           |
+| brokerId             | 0                              | broker id, 0 means master, positive integers mean slave                                                 |
+| storePathCommitLog                      | $HOME/store/commitlog/                              | file path for commit log                                                 |
+| storePathConsumerQueue                   | $HOME/store/consumequeue/                              | file path for consume queue                                              |
+| mapedFileSizeCommitLog     | 1024 * 1024 * 1024(1G) | mapped file size for commit log                                        |​ 
+| deleteWhen     | 04 | When to delete the commitlog which is out of the reserve time                                        |​ 
+| fileReserverdTime     | 72 | The number of hours to keep a commitlog before deleting it                                        |​ 
+| brokerRole     | ASYNC_MASTER | SYNC_MASTER/ASYNC_MASTER/SLAVE                                        |​ 
+| flushDiskType     | ASYNC_FLUSH | {SYNC_FLUSH/ASYNC_FLUSH}. Broker of SYNC_FLUSH mode flushes each message onto disk before acknowledging producer. Broker of ASYNC_FLUSH mode, on the other hand, takes advantage of group-committing, achieving better performance.                                        |​
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