Adds README to Declarative Shadow DOM example.

Refs #38.

Forgot to include this with the initial implementation.

examples/declarative_shadow_dom/README.md

@@ -0,0 +1,14 @@
+# Declarative Shadow DOM
+
+An example which uses
+[Declarative Shadow DOM](https://web.dev/declarative-shadow-dom/) to author
+components with strong style isolation.
+
+This uses the `@npm//rules_prerender:declarative_shadow_dom` component
+dependency to easily inject the Declarative Shadow DOM polyfill into the
+generated web page.
+
+The example also accepts input DOM and wraps it, however input DOM is insert as
+light DOM and placed with a `<slot></slot>` tag. This prevents the input from
+being included under the shadow root and having component-private styles applied
+to it. See [component.ts](./component.ts).