Add R Language tutorial in German (#9119)

* Add German R Ressource

* Update free-programming-books-de.md

books/free-programming-books-de.md

@@ -27,6 +27,7 @@
     * [Symfony](#symfony)
 * [Python](#python)
     * [Django](#django)
+* [R](#r)
 * [Ruby on Rails](#ruby-on-rails)
 * [Scilab](#scilab)
 * [Scratch](#scratch)
@@ -211,6 +212,11 @@
 * [Django Girls Tutorial](https://tutorial.djangogirls.org/de) (1.11) (HTML) (:construction: *in process*)
 
 
+#### R
+
+* [Einführung in R](https://methodenlehre.github.io/einfuehrung-in-R/) - Andrew Ellis, Boris Mayer (HTML)
+
+
 ### Ruby on Rails
 
 * [Praxiswissen Ruby](https://web.archive.org/web/20160328183933/https://oreilly.de/german/freebooks/rubybasger/pdf_rubybasger.pdf) - Sascha Kersken (PDF) *(:card_file_box: archived)*