Add Xero to company list (#1950)

Author: wing328

README.md

@@ -521,6 +521,7 @@ Here are some companies/projects (alphabetical order) using OpenAPI Generator in
 - [Telstra](https://dev.telstra.com)
 - [TUI InfoTec GmbH](http://www.tui-infotec.com/)
 - [unblu inc.](https://www.unblu.com/)
+- [Xero](https://www.xero.com/)
 - [Zalando](https://www.zalando.com)
 
 ## [5 - Presentations/Videos/Tutorials/Books](#table-of-contents)