Added "Julia language: a concise tutorial" (#2556)

* Added "Julia language: a concise tutorial"

https://www.gitbook.com/book/sylvaticus/julia-language-a-concise-tutorial

* Removed PDF tag and added the second blank line

free-programming-books.md

@@ -1443,6 +1443,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 
 * [Introducing Julia wikibook](https://en.wikibooks.org/wiki/Introducing_Julia)
 * [Julia by Example](http://samuelcolvin.github.io/JuliaByExample) - Samuel Colvin (GitHub repo)
+* [Julia language: a concise tutorial](https://www.gitbook.com/book/sylvaticus/julia-language-a-concise-tutorial) - Antonello Lobianco (GitBook)
 * [Learn Julia in Y minutes](https://learnxinyminutes.com/docs/julia) - Leah Hanson
 * [Programming in Julia](http://quant-econ.net/jl/learning_julia.html) - Thomas J. Sargent and John Stachurski
 * [The Julia Express](http://bogumilkaminski.pl/files/julia_express.pdf) - Bogomił Kamiński (PDF)