Updating readme templates for newly created projects (#24725)

We hypothesize that many first-time users creating a new project will look at the README.md as a starting point for "what next". As we've updated our documentation particularly for new users, having better links to our codelabs and cookbooks will help them be productive.

packages/flutter_tools/templates/app/README.md.tmpl

@@ -4,5 +4,13 @@
 
 ## Getting Started
 
-For help getting started with Flutter, view our online
-[documentation](https://flutter.io/).
+This project is a starting point for a Flutter application.
+
+A few resources to get you started if this is your first Flutter project:
+
+- [Lab: Write your first Flutter app](https://flutter.io/docs/get-started/codelab)
+- [Cookbook: Useful Flutter samples](https://flutter.io/docs/cookbook)
+
+For help getting started with Flutter, view our 
+[online documentation](https://flutter.io/docs), which offers tutorials, 
+samples, guidance on mobile development, and a full API reference.

packages/flutter_tools/templates/package/README.md.tmpl

@@ -4,6 +4,11 @@
 
 ## Getting Started
 
-For help getting started with Flutter, view our online [documentation](https://flutter.io/).
-
-For help on editing package code, view the [documentation](https://flutter.io/developing-packages/).
+This project is a starting point for a Dart
+[package](https://flutter.io/developing-packages/),
+a library module containing code that can be shared easily across
+multiple Flutter or Dart projects.
+
+For help getting started with Flutter, view our 
+[online documentation](https://flutter.io/docs), which offers tutorials, 
+samples, guidance on mobile development, and a full API reference.

packages/flutter_tools/templates/plugin/README.md.tmpl

@@ -4,7 +4,11 @@
 
 ## Getting Started
 
-For help getting started with Flutter, view our online
-[documentation](https://flutter.io/).
-
-For help on editing plugin code, view the [documentation](https://flutter.io/developing-packages/#edit-plugin-package).
+This project is a starting point for a Flutter
+[plug-in package](https://flutter.io/developing-packages/),
+a specialized package that includes platform-specific implementation code for
+Android and/or iOS.
+
+For help getting started with Flutter, view our 
+[online documentation](https://flutter.io/docs), which offers tutorials, 
+samples, guidance on mobile development, and a full API reference.