add german translation of README & CONTRIBUTING

[Walpurga03]

Summary

This Pull Request adds German translations of the README.md and CONTRIBUTING.md files. The new files, named README.de.md and CONTRIBUTING.de.md, contain the same information as the original files, translated into German.

Changes

• Added README.de.md with the German translation of the README.md content.
• Added CONTRIBUTING.de.md with the German translation of the CONTRIBUTING.md content.

Purpose

The purpose of this Pull Request is to provide German-speaking users with an easier understanding of the project by offering the documentation in their native language.

Screenshots

Before	After
N/A	N/A

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[crazywoola]

Hello I think you need to add the language badge in these files as well.

https://github.com/langgenius/dify/blob/main/README.md