Enhance documentation

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/advanced/multi-platform.md

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+# Multi-platform image
+
+You can build multi-platform images using the [`platforms` input](../../README.md#inputs) as described below.
+
+> :bulb: List of available platforms will be displayed and available through our [setup-buildx](https://github.com/docker/setup-buildx-action#about) action.
+
+> :bulb: If you want support for more platforms, you can use QEMU with our [setup-qemu](https://github.com/docker/setup-qemu-action) action.
+
+```yaml
+name: ci
+
+on:
+  push:
+    branches:
+      - 'master'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v2
+      -
+        name: Set up QEMU
+        uses: docker/setup-qemu-action@v1
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      -
+        name: Login to DockerHub
+        uses: docker/login-action@v1 
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+      -
+        name: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: user/app:latest
+```