Add note about Webpack 5 usage to changelog (#160)

Author: syvb

CHANGELOG.md

@@ -7,6 +7,7 @@
 * Breaking: dropped Svelte 2 support ([#150](https://github.com/sveltejs/svelte-loader/pull/150))
 * Breaking: dropped Node 8 support ([#157](https://github.com/sveltejs/svelte-loader/pull/157))
 * Add Webpack 5 support ([#151](https://github.com/sveltejs/svelte-loader/pull/151))
+    * Webpack 5 requires [an additional rule](https://github.com/sveltejs/svelte-loader#usage) in `webpack.config.js` to load the Svelte runtime correctly
 * Replace broken Svelte 2 HMR with the implementation from `rixo/svelte-loader-hot` ([#156](https://github.com/sveltejs/svelte-loader/pull/156))
 * Add Node 14 support and remove virtual modules implementation (fixes [#125](https://github.com/sveltejs/svelte-loader/pull/125), [#131](https://github.com/sveltejs/svelte-loader/pull/131))