docs(examples): new example using google.maps.visualization.HeatmapLa…

…yer (#445)

examples/heatmap/README.md

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+# Heatmap
+
+This uses the `useMapsLibrary` hook showing earthquake magnitude data in a
+heatmap.
+
+## Google Maps Platform API key
+
+This example does not come with an API key. Running the examples locally requires a valid API key for the Google Maps Platform.
+See [the official documentation][get-api-key] on how to create and configure your own key.
+
+The API key has to be provided via an environment variable `GOOGLE_MAPS_API_KEY`. This can be done by creating a
+file named `.env` in the example directory with the following content:
+
+```shell title=".env"
+GOOGLE_MAPS_API_KEY="<YOUR API KEY HERE>"
+```
+
+If you are on the CodeSandbox playground you can also choose to [provide the API key like this](https://codesandbox.io/docs/learn/environment/secrets)
+
+## Development
+
+Go into the example-directory and run
+
+```shell
+npm install
+```
+
+To start the example with the local library run
+
+```shell
+npm run start-local
+```
+
+The regular `npm start` task is only used for the standalone versions of the example (CodeSandbox for example)
+
+[get-api-key]: https://developers.google.com/maps/documentation/javascript/get-api-key