Add a changelog for Spectral Compressor

plugins/spectral_compressor/CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic
+Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [0.3.0] - 2023-01-15
+
+### Removed
+
+- The DC filter option is gone. It was used to prevent upwards compression from
+  amplifying DC and very low subbass signals in the original Spectral
+  Compressor, but this iteration of Spectral Compressor doesn't need it and the
+  feature only caused confusion.
+
+### Changed
+
+- The compression part of Spectral Compressor has been rewritten to have
+  theoretically smoother and cleaner transfer curves and to be slightly more
+  performant.
+- The downwards hi-freq rolloff parameter now correctly scales the ratios.
+  Previously the parameter didn't do much.