add link to Issues on Github

Author: Vladimir Kotal

intro.tex

@@ -48,7 +48,8 @@
 \item The source code files can be found on GitHub here:\\
 \url{https://github.com/devnull-cz/unix-linux-prog-in-c-src}
 \item In case you find any errors either in the text or in the example programs,
-we appreciate if you let us know.
+we appreciate if you let us know. Especially do not hesitate to create new
+issues on \url{https://github.com/devnull-cz/unix-linux-prog-in-c/issues}.
 \end{itemize}
 
 \pagebreak