Add README note to revealjs as we include an external dependency.

examples/revealjs/README.md

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+# RevealJS Demo
+
+The RevealJS demo uses an external extension called `{quarto-countdown}`.
+In addition to installing the `{quarto-pyodide}` extension, you must also install the `countdown` shortcode extension.
+
+You can do this by running in your Quarto Project:
+
+```sh
+# Install `quarto-countdown`
+quarto add gadenbuie/countdown/quarto
+
+# Install `quarto-pyodide`
+quarto add coatless-quarto/pyodide
+```