Added Think Perl 6 by Allen Downey (#2374)

* Added Think Perl 6 by Allen Downey

* Added author's name as suggested by @eshellman

free-programming-books.md

@@ -1708,6 +1708,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 
 * [Perl 6 Introduction](http://perl6intro.com)
 * [Perl 6 Programming](https://en.wikibooks.org/wiki/Perl_6_Programming)
+* [Think Perl 6](http://greenteapress.com/thinkperl6/thinkperl6.pdf) - Allen Downey (PDF)
 * [Using Perl 6](https://github.com/perl6/book/) (work in progress)