add releases link

Author: vladak

README.md

@@ -4,6 +4,8 @@ This project contains source code for the material that is used to lecture
 "Unix/Linux Programming in C" (NSWI015) class at the Faculty of Mathematics and
 Physics, Charles University in Prague.
 
+To get the PDFs, go to [Releases](https://github.com/devnull-cz/unix-linux-prog-in-c/releases).
+
 It is written in LaTeX.  See the `Makefile` on how to build the PDF document.
 
 The class home page is https://devnull-cz.github.io/unix-linux-prog-in-c/