add an entry to the BREAKING_CHANGES list

BREAKING_CHANGES.md

@@ -17,6 +17,7 @@ This is a comprehensive list of the breaking changes introduced in the major ver
   * [Legacy Browser Support Fields Deprecated](#legacy-browser-support-fields-deprecated)
     * [`dynamicImportShim`](#dynamicimportshim)
     * [`cssVarShim`](#cssvarshim)
+    * [`shadowDomShim`](#shadowdomshim)
   * [Deprecated `assetsDir` Removed from `@Component()` decorator](#deprecated-assetsdir-removed-from-component-decorator)
   * [Drop Node 12 Support](#drop-node-12-support)
   * [Strongly Typed Inputs](#strongly-typed-inputs)
@@ -118,6 +119,26 @@ export const config: Config = {
 };
 ```
 
+##### `shadowDomShim`
+
+If `extras.shadowDomShim` is set to `true` the Stencil runtime will check
+whether a shim for [shadow
+DOM](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_shadow_DOM)
+is needed in the current browser, and include one if so. For Stencil v3.0.0
+this field is renamed to `__deprecated__shadowDomShim`. To retain the previous
+behavior the new option can be set in your project's `stencil.config.ts`:
+
+```ts
+// stencil.config.ts
+import { Config } from '@stencil/core';
+
+export const config: Config = {
+  extras: {
+    __deprecated__shadowDomShim: true
+  }
+};
+```
+
 #### Deprecated `assetsDir` Removed from `@Component()` decorator
 The `assetsDir` field was [deprecated in Stencil v2.0.0](#componentassetsdir), but some backwards compatibility was retained with a warning message.
 It has been fully removed in Stencil v3.0.0 in favor of `assetsDirs`.