[AD] Add note

semester2/algorithms-and-probability/parts/combinatorics.tex

@@ -1,6 +1,8 @@
 \newsection
 \section{Combinatorics}
 \subsection{Introduction}
+Please note: This section was not part of the book and is written in very simple terms (it is taken from a summary I wrote 4 years ago during Gymnasium)
+
 Combinatorics was developed from the willingness of humans to gamble and the fact that everybody wanted to win as much money as possible.
 
 \subsection{Simple counting operations}