docs: Add the link to Apache RocketMQ MQTT in the readme.md

[XiaoyiPeng]

Make sure set the target branch to develop

What is the purpose of the change

For #3970

Brief changelog

Add the link to Apache RocketMQ MQTT in the readme.md

Verifying this change

XXXX

[codecov-commenter]

Codecov Report

Merging #3971 (ce4e0a2) into develop (40d9505) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop    #3971      +/-   ##
=============================================
- Coverage      47.68%   47.67%   -0.01%     
+ Complexity      4972     4970       -2     
=============================================
  Files            633      633              
  Lines          42579    42579              
  Branches        5589     5589              
=============================================
- Hits           20305    20301       -4     
- Misses         19764    19771       +7     
+ Partials        2510     2507       -3     

Impacted Files	Coverage Δ
...ache/rocketmq/common/stats/MomentStatsItemSet.java	39.13% <0.00%> (-4.35%)	⬇️
...ava/org/apache/rocketmq/filter/util/BitsArray.java	59.82% <0.00%> (-2.57%)	⬇️
...a/org/apache/rocketmq/store/StoreStatsService.java	35.85% <0.00%> (-1.69%)	⬇️
...ketmq/common/protocol/body/RegisterBrokerBody.java	83.69% <0.00%> (-1.09%)	⬇️
...ent/impl/consumer/DefaultLitePullConsumerImpl.java	68.79% <0.00%> (-0.52%)	⬇️
...ain/java/org/apache/rocketmq/store/MappedFile.java	51.40% <0.00%> (-0.36%)	⬇️
...etmq/client/latency/LatencyFaultToleranceImpl.java	50.00% <0.00%> (ø)
...mq/client/impl/producer/DefaultMQProducerImpl.java	45.47% <0.00%> (+0.37%)	⬆️
...n/java/org/apache/rocketmq/store/ha/HAService.java	72.33% <0.00%> (+0.66%)	⬆️
...org/apache/rocketmq/store/ha/WaitNotifyObject.java	73.21% <0.00%> (+7.14%)	⬆️
... and 1 more

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[coveralls]

Coverage decreased (-0.02%) to 51.741% when pulling ce4e0a2 on XiaoyiPeng:issue#3970 into 40d9505 on apache:develop.

[coveralls]

Coverage decreased (-0.02%) to 51.741% when pulling ce4e0a2 on XiaoyiPeng:issue#3970 into 40d9505 on apache:develop.