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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。

* 增加《Redis 设计与实现》书籍推荐。全书的讲解通俗易懂，知识点全面。读这本书，既能复习数据结构基本知识点，也能深入了解 Redis 底层运行原理。
* 《大型网站技术架构》这本书，里面基本没谈及 Redis 相关知识，而《Redis 设计与实现》 恰是一本很好的关于 Redis 的书籍。

Fix wrong link, see https://github.com/yanglbme

* Fix typo in redis-sentinel.md
* Thanks @tccbest 

update configuraiton to configuration

[ImgBot] Optimize images

*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。

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For details, see https://github.com/doocs/intro

倒排索引的一些补充说明

推荐《大型网站技术架构_核心原理与案例分析——李智慧》

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

add additional method to handle distributed session

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