[i18n-de] Translate README.md to German (#28933)

* Translate README.md to German

* Add links to README_de.md

* Remove invisible characters in README

* Change to a formal tone and fix punctuation marks

README_fr.md

@@ -56,6 +56,7 @@ limitations under the License.
         <a href="https://github.com/huggingface/transformers/blob/main/README_pt-br.md">Рortuguês</a> |
         <a href="https://github.com/huggingface/transformers/blob/main/README_te.md">తెలుగు</a> |
         <b>Français</b> |
+        <a href="https://github.com/huggingface/transformers/blob/main/README_de.md">Deutsch</a> |
     </p>
 </h4>