Adding code maintainability badge

Dear Contributors,

As professional developers, you probably must be well aware of the meaning and importance of code maintainability! It expresses how difficult and risky it is to change the code. Maintainability is affected by a variety of low-level code quality attributes, like source code metrics, code duplications, clone smells, coding issues, and so on. Being able to measure it is crucial for keeping it at desired level.

We, at FrontEndART, have developed advanced algorithms based on benchmarks and quality models that allow us to quantify and track changes of code maintainability. The approach called "Probabilistic Software Quality Model" is based on the research conducted at the Software Engineering Department of University of Szeged (Hungary): https://ieeexplore.ieee.org/document/6080791.

We are excited to allow open source communities to use our tool called QualityGate (https://www.quality-gate.com), which incorporates these methods, enabling them to track their projects' maintainability.

If you agree that measuring, monitoring and promoting the maintainability of your project's code base adds a value to it, please accept this pull-request, which just adds a new badge to your project that shows its actual level of maintainability.

If you have further questions or comments, please do not hesitate to contact us at info@frontendart.com.

Best regards,
FrontEndART team

README.md

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 | [:pencil2:](#pencil2-算法) | [:computer:](#computer-操作系统) | [:cloud:](#cloud-网络) | [:art:](#art-面向对象) | [:floppy_disk:](#floppy_disk-数据库) |[:coffee:](#coffee-java)| [:bulb:](#bulb-系统设计) |[:wrench:](#wrench-工具)| [:watermelon:](#watermelon-编码实践) |[:memo:](#memo-后记)|
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+[![Maintainability](https://cloud.quality-gate.com/dashboard/api/badge?projectName=CyC2018_CS-Notes&branchName=master)](https://cloud.quality-gate.com/dashboard/branches/135613#overview)
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