[ROCKETMQ-148] Polish the contributing guide.
[ROCKETMQ-148] Polish the contributing guide.

DR-YangLong/RocketMQ,Vansee/RocketMQ,lindzh/incubator-rocketmq,penghuazhou/incubator-rocketmq,vsair/incubator-rocketmq,vsair/incubator-rocketmq,shroman/incubator-rocketmq,lindzh/incubator-rocketmq,vesense/incubator-rocketmq,DR-YangLong/RocketMQ,penghuazhou/incubator-rocketmq,shroman/incubator-rocketmq,shroman/incubator-rocketmq,lindzh/incubator-rocketmq,penghuazhou/incubator-rocketmq,vsair/incubator-rocketmq,vesense/incubator-rocketmq,vesense/incubator-rocketmq