Update the doc description of why-dubbo.
By the way, remember to refresh your cache of page.

*Total -- 32.53kb -> 16.05kb (50.66%)

/images/zookeeper-distributed-lock.png -- 16.26kb -> 8.02kb (50.66%)
/docs/distributed-system/images/zookeeper-distributed-lock.png -- 16.26kb -> 8.02kb (50.66%)

*Total -- 58.20kb -> 26.53kb (54.42%)

/images/dubbo-not-keep-connection.png -- 11.80kb -> 4.77kb (59.59%)
/docs/distributed-system/images/dubbo-not-keep-connection.png -- 11.80kb -> 4.77kb (59.59%)
/images/dubbo-keep-connection.png -- 17.30kb -> 8.49kb (50.9%)
/docs/distributed-system/images/dubbo-keep-connection.png -- 17.30kb -> 8.49kb (50.9%)

consumer -> provider

* Update database-shard.md
* Update dubbo-operating-principle.md

Format docs and add some new content

Welcome to join the Doocs Open Source organization, see https://github.com/doocs/intro for details.

msater -> master

* Fix typos
* Update docs description

* Update docs description in ElasticSearch
* Welcome to join the WeChat Group, see https://github.com/doocs/intro/issues/6 for details.

* 更新 es 架构文章描述。
* 更新 README，欢迎所有面试官和面试者们来 #9 分享自己的想法。在这里对 @blackdog1987 表示感谢，他给我们分享了他自己作为技术面试官招人的一些侧重点和对面试者的建议。

update configuraiton to configuration

*Total -- 163.03kb -> 68.60kb (57.92%)

/images/redis-single-thread-model.png -- 81.52kb -> 34.30kb (57.92%)
/docs/high-concurrency/images/redis-single-thread-model.png -- 81.52kb -> 34.30kb (57.92%)

* 更新 redis 单线程模型文章知识点描述；
* 重新绘图，更清晰直观；
* 学习的时候，参照文字描述与图片，更加容易理解。

倒排索引的一些补充说明

Update cobar desc

- Update RocketMQ desc
- Fix #40 @rainzhao

- Fix typo in why-cache.md
- Close #39 @jiaxingzheng

- Fix typo in redis-persistence
- Close #37

Small changes in `dubbo-load-balancing.md`

- Fix typo in distributed-session
- Close #34 @chenlang888881

- Indices created in Elasticsearch 6.0.0 or later may only contain a single mapping type. Indices created in 5.x with multiple mapping types will continue to function as before in Elasticsearch 6.x. Mapping types will be completely removed in Elasticsearch 7.0.0.
- Fix #33
- Thank you @jptangchina

- Fix #32
- Thank you @CavaliersFor

*Total -- 56.62kb -> 24.34kb (57.01%)

/images/database-id-sequence-step.png -- 28.31kb -> 12.17kb (57.01%)
/docs/high-concurrency/images/database-id-sequence-step.png -- 28.31kb -> 12.17kb (57.01%)

更新分库分表后如何生成唯一ID

*Total -- 46.21kb -> 20.38kb (55.89%)

/images/mq-10.png -- 23.11kb -> 10.19kb (55.89%)
/docs/high-concurrency/images/mq-10.png -- 23.11kb -> 10.19kb (55.89%)

- 修改 Kafka 图片小错误
- 调整图片展示位置