Add a comment to mention attachment_on_the_fly gem

Closes thoughtbot#105

README.md

@@ -188,9 +188,12 @@ or more or the processors, and they are expected to ignore it.
 _NOTE: Because processors operate by turning the original attachment into the
 styles, no processors will be run if there are no styles defined._
 
-If you're interested in caching your thumbnail's width, height and size in the 
+If you're interested in caching your thumbnail's width, height and size in the
 database, take a look at the [paperclip-meta](https://github.com/y8/paperclip-meta) gem.
 
+Also, if you're interesting to generate the thumbnail on-the-fly, you might want
+to look into the [attachment_on_the_fly](https://github.com/drpentode/Attachment-on-the-Fly) gem.
+
 Events
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