docs: Add command API to docs and extension dev page.

docs/api/commands.rst

@@ -0,0 +1,9 @@
+.. _commands-api:
+
+************
+Commands API
+************
+
+.. automodule:: mopidy.commands
+    :synopsis: Commands API for Mopidy CLI.
+    :members:

docs/api/index.rst

@@ -13,6 +13,7 @@ API reference
     core
     audio
     frontends
+    commands
     ext
     config
     http

docs/extensiondev.rst

@@ -305,6 +305,10 @@ This is ``mopidy_soundspot/__init__.py``::
             from .backend import SoundspotBackend
             return [SoundspotBackend]
 
+        def get_command(self):
+            from .commands import SoundspotCommand
+            return SoundspotCommand()
+
         def register_gstreamer_elements(self):
             from .mixer import SoundspotMixer
             gobject.type_register(SoundspotMixer)
@@ -374,6 +378,34 @@ details.
         # Your backend implementation
 
 
+Example command
+===============
+
+If you want to extend the Mopidy with a new helper not run from the server,
+such as scanning for media, adding a command is the way to go. Your top level
+command name will always match your extension name, but you are free to add
+sub-commands with names of your choosing.
+
+The skeleton of a commands would look like this. See :ref:`command-api` for more
+details.
+
+::
+
+    from mopidy import commands
+
+
+    class SoundspotCommand(commands.Command):
+        help = 'Some text that will show up in --help'
+
+        def __init__(self):
+            super(SoundspotCommand, self).__init__()
+            self.add_argument('--foo')
+
+        def run(self, args, config, extensions):
+           # Your backend implementation
+           return 0
+
+
 Example GStreamer element
 =========================