Polish the contributing guide
Polish the contributing guide

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