docs: update package example

Author: incendial

example/README.md

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+# Examples
+
+Dart Code Metrics is a static analysis tool and code from it is not expected to be used directly. Instead, it should be added as a dev dependency and installed [as described here](https://dartcodemetrics.dev/docs/getting-started/installation).
+
+## Plugin and CLI
+
+To use Dart Code Metrics as a plugin or a CLI, check out [this repository](https://github.com/dart-code-checker/dart-code-metrics-example) with the example Flutter app. It covers DCM setup and shows all commands output.
+
+## As a library
+
+Dart Code Metrics can be used as a library, imported and called directly from your code. Continue with [this example](https://github.com/dart-code-checker/dart-code-metrics/blob/master/example/example.dart) in order to get more details.
+
+## Presets
+
+Dart Code Metrics supports presets:
+
+- all Dart rules [dart_all.yaml](https://github.com/dart-code-checker/dart-code-metrics/blob/master/lib/presets/dart_all.yaml)
+- all Flutter rules [flutter_all.yaml](https://github.com/dart-code-checker/dart-code-metrics/blob/master/lib/presets/flutter_all.yaml)
+- all rules [all.yaml](https://github.com/dart-code-checker/dart-code-metrics/blob/master/lib/presets/all.yaml)
+
+Presets can be enabled with `extends` config, more details [can be found here](https://dartcodemetrics.dev/docs/getting-started/configuration#extending-an-existing-configuration-preset).