Added a bash tutorial in free-courses-en.md (#1983)

* Added a bash tutorial in free-courses-en.md

* Missing PDF indication

free-courses-en.md

@@ -4,6 +4,7 @@
 * [Android](#android)
 * [AngularJS](#angularjs)
 * [Assembly](#assembly)
+* [Bash](#bash)
 * [C](#c)
 * [C#](#c-sharp)
 * [C++](#c-2)
@@ -61,6 +62,11 @@
 * [Shaping up with Angular.js](https://www.codeschool.com/courses/shaping-up-with-angular-js)
 
 
+### Bash
+
+* [Bash tutorial](http://gdrcorelec.ups-tlse.fr/files/bash.pdf) (PDF)
+
+
 ### C
 
 * [Introduction to Computer Science](https://www.edx.org/course/introduction-computer-science-harvardx-cs50x#.VNsjDvmUcmQ)