Add introductory paragraph to german translation

[TizianKurz]

I realized that other translations had an introductory paragraph and added it in the german version.

Fixes no open issue

Type of change

• [X] Other (please elaborate)

Checklist:

• [x] My content follows the style guidelines of this project
• [X] I have performed a self-review of my own content

[LantingHe]

这是来自QQ邮箱的假期自动回复邮件。你好，我最近正在休假中，无法亲自回复你的邮件。我将在假期结束后，尽快给你回复。

[roblarsen]

The other translations are out of date. The German translation is up to date:

https://github.com/h5bp/Front-end-Developer-Interview-Questions/pull/894

[TizianKurz]

Okay, thanks for your time.