deprecate 'backend' (singular) option

README.rst

@@ -120,8 +120,6 @@ it's own configuration, stored in a key of it's own name. For example:
 
 Note: versions prior to v1.0.0 used JSON instead of YAML.
 
-Note: v0.2.1 and older supported only a single backend, to support old configuration files ntfy will still accept a single backend (a string instead of a list of strings) with the key 'backend'.
-
 The available backends are in `ntfy.backends <ntfy.backends.html>`_,
 include only the module name in your config (eg: ``pushbullet`` not
 ``ntfy.backends.pushbullet``).

ntfy/config.py

@@ -43,6 +43,8 @@ def load_config(config_path=DEFAULT_CONFIG):
             logger.warning("Both 'backend' and 'backends' in config, "
                            "ignoring 'backend'.")
         else:
+            logger.warning(
+                "The 'backend' config option is deprecated, use 'backends'")
             config['backends'] = [config['backend']]
 
     return config