:memo: add PR template [skip ci]

[jiro4989]

PRのテンプレートを作成しました。
PRを出した方のテストが失敗して、直し方がわからない方の助力になるかと思います。

Check list

• [ ] Were all tests passed of CI?
  • (Case: failing test) Please run tests and fix your code (see also: Testing)
  • (Case: failing format) Please run ./linter.sh format-save (see also: CodeStyle and lint)
  • (Case: failing lint) Please run linter and fix your code (see also: CodeStyle and lint)
• [ ] (Case: adding new command) Were you added tests of your new commads?
• [ ] (Case: adding new command) Were you added descriptions of your new commands to README?

[greymd]

About issue and pull request templates のことですね。いいですね。