Update resources.html

I've added this line:
 
<li><a href="http://www.justinmind.com">Justinmind Prototyping Platform</a></li>
 
inside the "Post-PSD tools" section.  

I think this is the best fit for our Responsive Prototyping Platform: http://jimp.co/JustinmindPrototyper 

Thank you Brad!

  Cheers,

  Xavi.