document attachment helper

README.rdoc

@@ -999,6 +999,23 @@ The <tt>request.body</tt> is an IO or StringIO object:
     "Hello #{data['name']}!"
   end
 
+=== Attachments
+
+You can use the +attachment+ helper to tell the browser the response should be
+stored on disk rather than displayed in the browser.
+
+  get '/' do
+    attachment
+    "store it!"
+  end
+
+You can also pass it a file name:
+
+  get '/' do
+    attachment "info.txt"
+    "store it!"
+  end
+
 === Looking Up Template Files
 
 The <tt>find_template</tt> helper is used to find template files for rendering: