improve documentation for content_type, move into helpers section

README.rdoc

@@ -848,6 +848,20 @@ Similar to +body+, you can also set the status code and headers:
 Like +body+, +headers+ and +status+ with no arguments can be used to access
 their current values.
 
+=== Mime Types
+
+When using <tt>send_file</tt> or static files you may have mime types Sinatra
+doesn't understand. Use +mime_type+ to register them by file extension:
+
+  mime_type :foo, 'text/foo'
+
+You can also use it with the +content_type+ helper:
+
+  get '/' do
+    content_type :foo
+    "foo foo foo"
+  end
+
 === Generating URLs
 
 For generating URLs you should use the +url+ helper method, for instance, in
@@ -1216,17 +1230,6 @@ Or a range:
 Sinatra installs special <tt>not_found</tt> and <tt>error</tt> handlers when
 running under the development environment.
 
-== Mime Types
-
-When using <tt>send_file</tt> or static files you may have mime types Sinatra
-doesn't understand. Use +mime_type+ to register them by file extension:
-
-  mime_type :foo, 'text/foo'
-
-You can also use it with the +content_type+ helper:
-
-  content_type :foo
-
 == Rack Middleware
 
 Sinatra rides on Rack[http://rack.rubyforge.org/], a minimal standard