Add Upgrading note

UPGRADING

+##################################################
+#  NOTE FOR UPGRADING FROM PRE-3.0 VERSION       #
+##################################################
+
+Paperclip 3.0 introduces a non-backward compatible change in your attachment
+path. This will help to prevent attachment name clashes when you have
+multiple attachments with the same name. If you didn't alter your
+attachment's path and are using Paperclip's default, you'll have to add
+`:path` and `:url` to your `has_attached_file` definition. For example:
+
+    has_attached_file :avatar,
+      :path => ":rails_root/public/system/:attachment/:id/:style/:filename",
+      :url => "/system/:attachment/:id/:style/:filename"
+

paperclip.gemspec

@@ -18,6 +18,10 @@ Gem::Specification.new do |s|
   s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   s.require_paths = ["lib"]
 
+  if File.exists?('UPGRADING')
+    s.post_install_message = File.read("UPGRADING")
+  end
+
   s.requirements << "ImageMagick"
 
   s.add_dependency('activerecord', '>= 3.0.0')