docs: add dev importer and cli text from wiki

docs/dev/cli.rst

@@ -0,0 +1,9 @@
+Providing a CLI
+===============
+
+The ``beets.ui`` module houses interactions with the user via a terminal, the
+:doc:`/reference/cli`.
+The main function is called when the user types beet on the command line.
+The CLI functionality is organized into commands, some of which are built-in
+and some of which are provided by plugins. The built-in commands are all
+implemented in the ``beets.ui.commands`` submodule.

docs/dev/importer.rst

@@ -0,0 +1,19 @@
+Music Importer
+==============
+
+The importer component is responsible for the user-centric workflow that adds
+music to a library. This is one of the first aspects that a user experiences
+when using beets: it finds music in the filesystem, groups it into albums,
+finds corresponding metadata in MusicBrainz, asks the user for intervention,
+applies changes, and moves/copies files. A description of its user interface is
+given in :doc:`/guides/tagger`.
+
+The workflow is implemented in the ``beets.importer`` module and is
+distinct from the core logic for matching MusicBrainz metadata (in the
+``beets.autotag`` module). The workflow is also decoupled from the command-line
+interface with the hope that, eventually, other (graphical) interfaces can be
+bolted onto the same importer implementation.
+
+The importer is multithreaded and follows the pipeline pattern. Each pipeline
+stage is a Python coroutine. The ``beets.util.pipeline`` module houses
+a generic, reusable implementation of a multithreaded pipeline.

docs/dev/index.rst

@@ -13,3 +13,5 @@ and write metadata tags in media files.
 
     plugins
     library
+    importer
+    cli