Add german book list to the README

Author: Wilhelm Babernits

README.md

@@ -9,6 +9,8 @@ Moved to GitHub for collaborative updating and for the purpose of reSRC.io : [Li
 #### [French : Free programming books](http://resrc.io/list/33/livres-gratuits-sur-la-programmation/) -- free-programming-books-fr.md
 The French list was based on <http://progdupeu.pl/forums/sujet/43/une-banque-de-liens>.
 
+#### German : Free programming books -- free-programming-books-de.md
+
 #### Italian : Free programming books -- free-programming-books-it.md
 
 #### Russian : Free programming books -- free-programming-books-ru.md