mention playback feature in upgrade guide

UPGRADE.md

@@ -2,6 +2,10 @@ This guide will help you update your code when upgrading from older versions of
 
 # rodio 0.20.1 or earlier to current GitHub version
 
+## Features
+- If you use disable the rodio features with `default_features = false` in
+  `Cargo.toml` you need to add a new feature `playback`.
+
 ## Source implementations
 - Source had a required method `current_frame_len`. In the latest version of rodio *frame* has been renamed to *span*. You will need to change every occurrence of `current_frame_len` to `current_span_len`.