docs: Add ImmutableObject.copy() example

docs/api/models.rst

@@ -7,6 +7,21 @@ backends and between the backends and the MPD frontend. All fields are optional
 and immutable. In other words, they can only be set through the class
 constructor during instance creation.
 
+If you want to modify a model, use the
+:meth:`~mopidy.models.ImmutableObject.copy` method. It accepts keyword
+arguments for the parts of the model you want to change, and copies the rest of
+the data from the model you call it on. Example::
+
+    >>> from mopidy.models import Track
+    >>> track1 = Track(name='Christmas Carol', length=171)
+    >>> track1
+    Track(artists=[], length=171, name='Christmas Carol')
+    >>> track2 = track1.copy(length=37)
+    >>> track2
+    Track(artists=[], length=37, name='Christmas Carol')
+    >>> track1
+    Track(artists=[], length=171, name='Christmas Carol')
+
 
 Data model relations
 ====================