[Tails-l10n] (review)[de] wiki/src/doc/first_steps/installat…

Hi,
I translated the installation page. Please review:

https://gitlab.com/spriver/tails
branch: de_installation
latest commit: 8a060060 Translated missing strings, corrections


Please also do not forget to review the "startup option" translation I
made on 2015-04-25. Just as a reminder, it has been a couple of days
since then (:


Cheers,

spriver