Add notes and example re importing from Leaflet ESM

## Usage

Till the documentation is ready please check the [demo project](https://github.com/vue-leaflet/vue3-demo-project/blob/master/src/App.vue)

Until the complete documentation is ready, please check the [demo project](https://github.com/vue-leaflet/vue3-demo-project/blob/master/src/App.vue) for example usage.

### Working with Leaflet

> **N.B.** Using `import L from "leaflet"` or `import { ... } from "leaflet"` can lead to unexpected errors.

To provide server-side rendering and tree-shaking capabilities, vue-leaflet uses async imports from the Leaflet ESM.

This can lead to issues when importing additional methods from Leaflet, because the two instances of the Leaflet

classes are technically no longer the same. See [Issue 48](https://github.com/vue-leaflet/vue-leaflet/issues/48) for more.

To avoid these issues, import any Leaflet methods asynchronously in response to the LMap component's `@ready` event:

```vue

<template>

  <l-map>

    <l-geo-json :geojson="geojson" :options="geojsonOptions" />

  </l-map>

</template>

<script>

// DON'T load Leaflet components here!

import { LMap, LGeoJson } from "./../../components";

export default {

  components: {

    LMap,

    LGeoJson,

  },

  data() {

    return {

      geojson: {

        type: "FeatureCollection",

        features: [

          // ...

        ],

      },

      geojsonOptions: {

        // Options that don't rely on Leaflet methods.

      },

    };

  },

  async beforeMount() {

    // HERE is where to load Leaflet components!

    const { circleMarker } = await import("leaflet/dist/leaflet-src.esm");

    // And now the Leaflet circleMarker function can be used by the options:

    this.geojsonOptions.pointToLayer = (feature, latLng) =>

      circleMarker(latLng, { radius: 8 });

    this.mapIsReady = true;

  },

};

</script>

```

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