changelog: new r128_targetlevel option

Add changelog entries for the introduction of the `r128_targetlevel`
configuration option, superseding the `method` option.

docs/changelog.rst

@@ -42,6 +42,13 @@ New features:
   new ``discogs_albumid`` field from the Discogs API.
   Thanks to :user:`thedevilisinthedetails`.
   :bug:`465` :bug:`3322`
+* :doc:`plugins/replaygain`: ``r128_targetlevel`` is a new configuration option
+  for the ReplayGain plugin: It defines the reference volume for files using
+  ``R128_`` tags. ``targtelevel`` only configures the reference volume for
+  ``REPLAYGAIN_`` files.
+  This also deprecates the ``bs1770gain`` ReplayGain backend's ``method``
+  option. Use ``targetlevel`` and ``r128_targetlevel`` instead.
+  :bug:`3065`
 
 Fixes:
 
@@ -86,6 +93,10 @@ For plugin developers:
 * There were sporadic failures in ``test.test_player``. Hopefully these are
   fixed. If they resurface, please reopen the relevant issue.
   :bug:`3309` :bug:`3330`
+* The internal structure of the replaygain plugin had some changes: There are no
+  longer separate R128 backend instances. Instead the targetlevel is passed to
+  ``compute_album_gain`` and ``compute_track_gain``.
+  :bug:`3065`
 
 For packagers: