Add changelog entry

UNRELEASED.md

@@ -32,6 +32,7 @@ Use [the changelog guidelines](https://git.io/polaris-changelog-guidelines) to f
 - Renamed `yarn run ts` to `yarn run type-check` to match most other Shopify projects
 - Fixed deprecation notice in build ([#1754](https://github.com/Shopify/polaris-react/pull/1754))
 - Simplified our rollup plugin for sass compilation while retaining identical behaviour ([#1753](https://github.com/Shopify/polaris-react/pull/1753))
+- Use explicit imports for our base sass mixins instead of having them implictly defined at build-time. This simplifes our build config and other tooling that wants to build us from source [[#1680](https://github.com/Shopify/polaris-react/pull/1680)]
 
 ### Dependency upgrades