Added some introductory installation instructions for Ubuntu

The installation instructions in this patch don’t work 100%, they get me this far:

    % ruby scraper.rb 
    
    scraper.rb:4:in `require': no such file to load -- nokogiri (LoadError)
        from scraper.rb:4

I can get a little further with the `-rubygems` flag to ruby, which I don’t understand but:

    % ruby -rubygems scraper.rb 

    scraper.rb:16:in `build_or_set_dir': undefined method `exists?' for Dir:Class (NoMethodError)
        from scraper.rb:26:in `download_pdfs'
        from /var/lib/gems/1.8/gems/nokogiri-1.5.6/lib/nokogiri/xml/node_set.rb:239:in `each'
        from /var/lib/gems/1.8/gems/nokogiri-1.5.6/lib/nokogiri/xml/node_set.rb:238:in `upto'
        from /var/lib/gems/1.8/gems/nokogiri-1.5.6/lib/nokogiri/xml/node_set.rb:238:in `each'
        from scraper.rb:25:in `download_pdfs'
        from scraper.rb:97