docs: Add the docc-generator preview command to README.md

Author: euanh

Sources/swift-container-plugin/README.md

@@ -3,4 +3,10 @@
 `ContainerImageBuilderPluginDocumentation` is an otherwise empty target that includes high-level,
 user-facing documentation about using the ContainerImageBuilder plugin from the command-line.
 
+To preview the documentation, run the following command:
+
+```bash
+swift package --disable-sandbox preview-documentation --target swift-container-plugin
+```
+
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