German translation exercises

[Maroel]

I'm new on GitHub and not too deep into CSS or SASS, but experienced some HTML and CSS ages ago (about 10 years ago I coded HTML). Since then on conceptual level. Translated already two english books (60 + 100 pages) into German.

[claudiopro]

This is awesome, @Maroel! I'll get to review your changes later today 😃

[claudiopro]

@Maroel thank you for your PR! It looks great so far 👍

There's only one missing bit left to complete. You should also translate the file https://github.com/workshopper/learn-sass/blob/master/footer.en.md. Without this file, the user will not see instructions to validate their submission, like the following:

## HINTS

  To make a Sass (SCSS) stylesheet, create a new file with a .scss extension
  and start writing SCSS. When you are done, you must run:

     $ learn-sass verify stylesheet.scss

  to proceed. Your stylesheet will be tested, a report will be generated,
  and the lesson will be marked 'completed' if you are successful.

Please create a https://github.com/workshopper/learn-sass/blob/master/footer.de.md containing the German translation of footer.en.md.

[claudiopro]

Thank you so much! I'll merge now

[claudiopro]

Implements #60