Add German translation

Adds link to the unofficial German translation of our documentation, which I crafted over the last few days and reorders the links by English version of the languages' names.

/to @mdo @cvrebert to review and check out. For now, the site features a "Coming up." message. I'll update my local version of the finished translation with upstream changes and launch the final product alongside v3.1.

http://holdirbootstrap.de / http://juthilo.github.io/bootstrap-german
http://github.com/juthilo/bootstrap-german