Make books location more explicit

Author: ilias-bikenov

README.md

@@ -4,6 +4,8 @@
 
 [Index EN](content/en/Index.md) | [Оглавление RU](content/ru/Index.md) | [Зміст UK](content/uk/Index.md) | [Obsah CZ](content/cz/Index.md)
 
+Generated books are located in */release* folder
+
 To build pdf file run:
 
 ```