Added link to opensource Bash book (#5217)

books/free-programming-books.md

@@ -1042,6 +1042,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Conquering the Command Line](http://conqueringthecommandline.com/book/frontmatter) - Mark Bates (HTML)
 * [Getting Started with BASH](http://www.hypexr.org/bash_tutorial.php) (HTML)
 * [Google Shell Style Guide](https://google.github.io/styleguide/shell.xml) - Paul Armstrong (HTML)
+* [Introduction to Bash Scripting](https://github.com/bobbyiliev/introduction-to-bash-scripting) - Bobby Iliev (Markdown, PDF)
 * [Introduction to the Command Line](https://launchschool.com/books/command_line) - Launch School (HTML)
 * [Linux Shell Scripting Tutorial - A Beginner's Handbook (2002)](http://www.freeos.com/guides/lsst/) - Vivek G. Gite (HTML)
 * [Linux Shell Scripting Tutorial (LSST) v2.0](https://bash.cyberciti.biz/guide/Main_Page) - Vivek Gite (HTML)