added a course Linux Operating System Fundamentals (#4035)

* added a e-book for Linux (PDF)

* add Linux Operating System Fundamentals

* updated

free-courses-en.md

@@ -322,6 +322,7 @@
 ### Linux
 
 * [Introduction To Linux](https://www.edx.org/course/introduction-to-linux/) - The Linux Foundation (edx)
+* [Linux Operating System Fundamentals](https://www.udemy.com/course/linux-academy-linux-operating-system-fundamentals/) - Linux Academy (Udemy)
 
 
 ### Lua