Added Sinatra book

free-programming-books.md

@@ -101,6 +101,7 @@
 * [Racket](#racket)
 * [Ruby](#ruby)
     * [RSpec](#rspec)
+    * [Sinatra](#sinatra)
 * [Ruby on Rails](#ruby-on-rails)
 * [Rust](#rust)
 * [Sage](#sage)
@@ -1087,6 +1088,9 @@ See also [TeX](#tex)
 ###RSpec
 * [Better Specs (RSpec Guidelines with Ruby)](http://betterspecs.org)
 
+###Sinatra
+* [Sinatra Book](https://github.com/sinatra/sinatra-book)
+
 ###Ruby on Rails
 * [Ruby on Rails Tutorial: Learn Rails By Example](http://ruby.railstutorial.org/ruby-on-rails-tutorial-book)
 * [Objects on Rails](http://objectsonrails.com)