add instruction to call pub get (dart-archive/lints#21)

Author: pq

pkgs/lints/README.md

@@ -39,11 +39,10 @@ For new apps created with `dart create`, the lints are enabled by default.
 
 For existing apps or packages, take these steps to enable these lints:
 
-1. Add a *dev* dependency on this package to your `pubspec.yaml` file:
+1. In a terminal, located at the root of your package, run this command:
 
-```yaml
-dev_dependencies:
-  lints: ^1.0.0
+```terminal
+dart pub add --dev lints
 ```
 
 2. Create a new `analysis_options.yaml` file, next to the pubspec, that includes