Update README.md

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* 核心理论为[Google Dapper论文：Dapper, a Large-Scale Distributed Systems Tracing Infrastructure](http://research.google.com/pubs/pub36356.html),英语有困难的同学可参考[国内翻译](http://duanple.blog.163.com/blog/static/70971767201329113141336/)

* 本分析系统能通过不修改或少量修改代码的模式，对现有的JAVA应用或J2EE应用进行监控和数据收集，并针对应用进场进行准实时告警。此外提供大量的调用性能分析功能，解决目前的监控系统主要监控进程、端口而非应用实际性能的问题。

* 支持国内常用的dubbo以及dubbox等常见RPC框架，支持应用异常的邮件告警

* skywalking-sdk层面提供的埋点API，同步阻塞访问时间小于100μs

* 通过[byte-buddy](https://github.com/raphw/byte-buddy)，部分插件将通过动态字节码机制，避免代码侵入性，完成监控。动态代码模式埋点，同步阻塞访问时间应在200-300μs

* SkyWalking is an open source Large-Scale Distributed Systems Tracing Infrastructure, also been known as APM(Application Performance Management) tool. SkyWalking provides a solution to help monitor and analysis a Large-Scale Distributed Systems.

* SkyWalking supports popular rpc frameworks, such as [dubbo](https://github.com/alibaba/dubbo), [dubbox](https://github.com/dangdangdotcom/dubbox), etc., supports email-alert when application occurs unexpected exception。

* SkyWalking's basic API, execution time of blocking saving span must less than 100μs.