added [Writing Shell Scripts] to [Bash]

free-programming-books.md

@@ -679,6 +679,7 @@
 * [Google Shell Style Guide](http://google-styleguide.googlecode.com/svn/trunk/shell.xml)
 * [Lhunath's Bash Guide](http://mywiki.wooledge.org/BashGuide)
 * [The Command Line Crash Course](http://learncodethehardway.org/cli/book/) (also a Powershell reference)
+* [Writing Shell Scripts](http://linuxcommand.org/lc3_writing_shell_scripts.php) - William E. Shotts, Jr.
 
 
 ### Basic