[ffigen] Suggest using `dart pub add -d ffigen` (#259)

pkgs/ffigen/README.md

diff --git a/pkgs/ffigen/README.md b/pkgs/ffigen/README.md
index 0cf4247..a4439bb 100644
--- a/pkgs/ffigen/README.md
+++ b/pkgs/ffigen/README.md
@@ -41,7 +41,7 @@
 }
 ```
 ## Using this package
-- Add `ffigen` under `dev_dependencies` in your `pubspec.yaml`.
+- Add `ffigen` under `dev_dependencies` in your `pubspec.yaml` (run `dart pub add -d ffigen`).
 - Install LLVM (see [Installing LLVM](#installing-llvm)).
 - Configurations must be provided in `pubspec.yaml` or in a custom YAML file (see [configurations](#configurations)).
 - Run the tool- `dart run ffigen`.