docs: note about action usage

Author: crazy-max

README.md

@@ -9,6 +9,14 @@
 GitHub Action to set up (download and install) [Docker CE](https://docs.docker.com/engine/).
 Works on Linux, macOS and Windows.
 
+> **Note**
+>
+> This action is useful if you want to pin against a specific Docker version or
+> set up a custom daemon configuration or if Docker is not available on your
+> runner. If you're using [GitHub-hosted runners](https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources)
+> on Linux or Windows, Docker is already up and running, so it might not be
+> necessary to use this action.
+
 ![Screenshot](.github/setup-docker-action.png)
 
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