Add decorators docs to pages lists.

docs/developers/index.rst

@@ -35,6 +35,7 @@ Following references cover everything you want to know about writing your own ap
    shortcuts
    views_errors
    forms
+   decorators
    settings
    context_processors
    mails

docs/index.rst

@@ -24,6 +24,7 @@ Table of Contents
    developers/shortcuts
    developers/views_errors
    developers/forms
+   developers/decorators
    developers/settings
    developers/context_processors
    developers/mails