Add RubyKaigi 2019 presentation (#2693)

* Add RubyKaigi 2019 presentation

I talked "How to use OpenAPI3 for API developer" in RubyKaigi 2019
These are the topics of OpenAPI Generator

* Add the conference name

Author: ota42y

README.md

@@ -566,6 +566,7 @@ Here are some companies/projects (alphabetical order) using OpenAPI Generator in
 - 2019-03-25 - [Access any REST service with the SAP S/4HANA Cloud SDK](https://blogs.sap.com/2019/03/25/integrate-sap-s4hana-cloud-sdk-with-open-api/) by [Alexander Duemont](https://people.sap.com/alexander.duemont)
 - 2019-03-25 - [OpenAPI generatorを試してみる](https://qiita.com/amuyikam/items/e8a45daae59c68be0fc8) by [@amuyikam](https://twitter.com/amuyikam)
 - 2019-03-27 - [OpenAPI3を使ってみよう！Go言語でクライアントとスタブの自動生成まで！](https://techblog.zozo.com/entry/openapi3/go) by [@gold_kou](https://twitter.com/gold_kou)
+- 2019-04-18 - [How to use OpenAPI3 for API developer (RubyKaigi 2019)](https://speakerdeck.com/ota42y/how-to-use-openapi3-for-api-developer) by [@ota42y](https://twitter.com/ota42y) at [RubyKaigi 2019](https://rubykaigi.org/2019)
 
 ## [6 - About Us](#table-of-contents)