docs: Update change log and local library info

docs/changelog.rst

@@ -15,6 +15,12 @@ v0.18.0 (UNRELEASED)
 - Add :class:`mopidy.models.Ref` class for use as a lightweight reference to
   other model types, containing just an URI, a name, and an object type.
 
+**Extension registry**
+
+- Switched to using a registry model for classes provided by extension. This
+  allows extensions to be extended as needed for plugable local libraries.
+  (Fixes :issue:`601`)
+
 **Pluggable local libraries**
 
 Fixes issues :issue:`44`, partially resolves :issue:`397`, and causes
@@ -33,6 +39,15 @@ a temporary regression of :issue:`527`.
 - Added support for deprecated config values in order to allow for
   graceful removal of :confval:`local/tag_cache_file`.
 
+- Added :confval:`local/library` to select which library to use.
+
+- Added :confval:`local/data_dir` to have a common setting for where to store
+  local library data. This is intended to avoid every single local library
+  provider having to have it's own setting for this.
+
+- Added :confval:`local/scan_flush_threshold` to control how often to tell
+  local libraries to store changes.
+
 **Streaming backend**
 
 - Live lookup of URI metadata has been added. (Fixes :issue:`540`)

docs/ext/local.rst

@@ -109,3 +109,9 @@ disable the current default library ``json``, replacing it with a third
 party one. When running :command:`mopidy local scan` mopidy will populate
 whatever the current active library is with data. Only one library may be
 active at a time.
+
+To create a new library provider you must create class that implements the
+:class:`~mopidy.backends.local.Libary` interface and install it in the
+extension registry under ``local:library``. Any data that the library needs
+to store on disc should be stored in :confval:`local/data_dir` using the
+library name as part of the filename or directory to avoid any conflicts.

mopidy/backends/local/__init__.py

@@ -134,7 +134,7 @@ class Library(object):
 
     def flush(self):
         """
-        Called for every n-th track indicating that work should be comitted.
+        Called for every n-th track indicating that work should be committed.
         Sub-classes are free to ignore these hints.
 
         :rtype: Boolean indicating if state was flushed.