docs: add link to a slideshare presentation (#4174)

Author: janweinschenker

README.md

@@ -643,6 +643,7 @@ Here are some companies/projects (alphabetical order) using OpenAPI Generator in
 - 2019-09-22 - [RESTful APIs: Tutorial of OpenAPI Specification](https://medium.com/@amirm.lavasani/restful-apis-tutorial-of-openapi-specification-eeada0e3901d) by [Amir Lavasani](https://medium.com/@amirm.lavasani)
 - 2019-09-22 - [Redefining SDKs as software diversity kits](https://devrel.net/dev-rel/redefining-sdks-as-software-diversity-kits) by [Sid Maestre (Xero)](https://twitter.com/sidneyallen) at [DevRelCon San Francisco 2019](https://sf2019.devrel.net/)
 - 2019-09-23 - [swaggerからOpenApi GeneratorでSpringのコードを自動生成](https://qiita.com/littleFeet/items/492df2ad68a0799a5e5e) by [@littleFeet](https://qiita.com/littleFeet) at [Qiita](https://qiita.com/)
+- 2019-09-24 - [Eine Stunde was mit Api First!](https://www.slideshare.net/JanWeinschenker/eine-stunde-was-mit-api-first) by [@janweinschenker](https://twitter.com/janweinschenker) at [Java Forum Nord](https://javaforumnord.de/)
 - 2019-10-09 - [openapi-generator で生成した Go クライアントで Bearer 認証をする](https://autopp-tech.hatenablog.com/entry/2019/10/09/222039) by [Akira Tanimura](https://github.com/autopp)
 - 2019-10-12 - [OpenApi自动生成client](https://blog.csdn.net/wxid2798226/article/details/102527467) by [郑泽洲](https://me.csdn.net/wxid2798226)
 - 2019-10-16 - [How to ship APIs faster?](https://medium.com/@accounts_76224/how-to-ship-apis-faster-cabef2f819e4) by [Simon Guilliams @ PoniCode](https://ponicode.dev)