Add a link to presentation at Open Source Summit Japan 2020 (#8596)

* Add a link to presentation at Open Source Summit Japan 2020

* rearrange

Author: wing328

README.md

@@ -804,6 +804,7 @@ Here are some companies/projects (alphabetical order) using OpenAPI Generator in
 - 2020-10-31 - [[B2] OpenAPI Specification으로 타입-세이프하게 API 개발하기: 희망편 VS 절망편](https://www.youtube.com/watch?v=J4JHLESAiFk) by 최태건 at [FEConf 2020](https://2020.feconf.kr/)
 - 2020-11-05 - [Automated REST-Api Code Generation: Wie IT-Systeme miteinander sprechen](https://www.massiveart.com/blog/automated-rest-api-code-generation-wie-it-systeme-miteinander-sprechen) by Stefan Rottensteiner at [MASSIVE ART Blog](https://www.massiveart.com/blog)
 - 2020-12-01 - [OpenAPI GeneratorでGoのAPIサーバー/クライアントコードを自動生成する](https://qiita.com/saki-engineering/items/b20d8b6074c4da9664a5) by [@saki-engineering](https://qiita.com/saki-engineering)
+- 2020-12-04 - [Scaling the Test Coverage of OpenAPI Generator for 30+ Programming Languages](https://www.youtube.com/watch?v=7Lke9dHRqT0) by [William Cheng](https://github.com/wing328) at [Open Source Summit Japan + Automotive Linux Summit 2020](https://events.linuxfoundation.org/archive/2020/open-source-summit-japan/) ([Slides](https://speakerdeck.com/wing328/scaling-the-test-coverage-of-openapi-generator-for-30-plus-programming-languages))
 - 2020-12-09 - [プロジェクトにOpenAPI Generatorで自動生成された型付きAPI Clientを導入した話](https://qiita.com/yoshifujiT/items/905c18700ede23f40840) by [@yoshifujiT](https://github.com/yoshifujiT)
 - 2020-12-15 - [Next.js + NestJS + GraphQLで変化に追従するフロントエンドへ 〜 ショッピングクーポンの事例紹介](https://techblog.yahoo.co.jp/entry/2020121530052952/) by [小倉 陸](https://github.com/ogugu9) at [Yahoo! JAPAN Tech Blog](https://techblog.yahoo.co.jp/)
 - 2021-01-08 - [Hello, New API – Part 1](https://www.nginx.com/blog/hello-new-api-part-1/) by [Jeremy Schulman](https://www.nginx.com/people/jeremy-schulman/) at [Major League Baseball](https://www.mlb.com)