Add @namusyaka to the Grape team

Author: namusyaka

team/index.md

@@ -20,6 +20,7 @@ Grape was created by [Michael Bleigh](https://github.com/mbleigh) at [Intridea](
 * [Dmitry Dedov, @dm1try](https://github.com/dm1try): Minsk, Belarus.
 * [Adam Gotterer, @adamgotterer](https://github.com/adamgotterer): New York, USA.
 * [Vladimir Kochnev, @marshall-lee](https://github.com/marshall-lee): Saint Petersburg, Russia.
+* [Kunpei Sakai, @namusyaka](https://github.com/namusyaka): Tokyo, Japan.
 
 #### [grape-entity](https://github.com/ruby-grape/grape-entity)