add basic changelog

Author: dominikschubert

CHANGELOG.md

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+# Changelog
+
+## [0.3.0]
+
+### Breaking Changes
+
+- **`use-pro` now defaults to `true`.** The action will use the `localstack/localstack-pro` image by default. Workflows that relied on the community image without setting `use-pro` will now get the Pro image, which requires a `LOCALSTACK_AUTH_TOKEN`. Set `use-pro: 'false'` explicitly if you need the community image. For newer versions of LocalStack these images are equivalent.
+
+### Deprecated
+
+- **`use-pro` input will be removed in a future release as we are consolidating our docker images**.
+