Update contributors.md

[yusuke]

Issue 番号 / Issue ID

• ドキュメント類のGitHub Pagesへの移行 #345

目的 / Purpose

contributorに追加。

破壊的変更をもたらしますか / Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request の種類 / Pull Request type

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

検証方法 / How to test

contributors.mdの表示を確認

確認事項 / What to check

その他 / Other information

[keiji]

LGTM!