docs: mention change to Symphonia in upgrade guide

Author: roderickvd

UPGRADE.md

@@ -3,14 +3,16 @@ 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`.
+- If you use disable the rodio features with `default_features = false` in `Cargo.toml` you need to
+  add a new feature `playback`.
+- The default decoders have changed to Symphonia, which itself is licensed under MPL. If you want
+  to revert to the old decoders, you need to enable the `claxon`, `hound` and `lewton` features in `Cargo.toml` for respectively FLAC, WAV and Ogg Vorbis.
 
 ## 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`.
 
 ## OutputStream
-- The outputstream is now more configurable. Where you used `OutputStream::try_default()` you have a choice:
+- The output stream is now more configurable. Where you used `OutputStream::try_default()` you have a choice:
     - *(recommended)* Get an error when the default stream could not be opened: `OutputStreamBuilder::open_default_stream()?`
     - Stay close to the old behavior using:
       `OutputStreamBuilder::open_stream_or_fallback()`, which tries to open the