Added Japanese proofreader

This MR is a request to add to the list of Japanese proofreaders.

Translation results

• https://gitlab.com/gitlab-jp/www-gitlab-jp/blob/master/data/release_posts/2019_04_23_gitlab_11_7_released.yml
• https://gitlab.com/gitlab-jp/www-gitlab-jp/blob/master/data/release_posts/2019_06_12_gitlab_11_8_released.yml
• https://gitlab.com/gitlab-jp/www-gitlab-jp/blob/master/data/release_posts/2019_07_30_gitlab_11_9_released.yml