[Doc/Userman] add clarification about creduce having to be installed

doc/userman/user-analysis-scripts.tex

@@ -312,6 +312,9 @@ different. It is available at\\
   described in a post from the \FramaC blog.
 \end{description}
 
+To use the \texttt{creduce.sh} script, you need to have the C-Reduce tool
+installed in your path or in environment variable \texttt{CREDUCE}.
+
 \section{Practical Examples: Open Source Case Studies}
 
 The {\em open-source-case-studies} Git repository (OSCS for short),