Initial documentation for the plugin package

pkg/analyzer_plugin/doc/tutorial/getting_started.md

+# Getting Started
+
+## Creating a Minimal Plugin
+
+To implement a plugin, start by creating a simple package and create a class
+that is a subclass of `ServerPlugin`. This class will need to implement a
+constructor, three getters, and two methods. The getters provide some basic
+information about your plugin: the name and version, both of which are included
+in error messages if there is a problem encountered, and a list of glob patterns
+for the files that the plugin cares about. The methods ...
+
+Here's an example of what a minimal plugin might look like.
+
+```dart
+class MyPlugin extends ServerPlugin {
+  MyPlugin(ResourceProvider provider) : super(provider);
+
+  @override
+  List<String> get fileGlobsToAnalyze => <String>['**/*.dart'];
+
+  @override
+  String get name => 'My fantastic plugin';
+
+  @override
+  String get version => '1.0.0';
+
+  @override
+  AnalysisDriverGeneric createAnalysisDriver(ContextRoot contextRoot) {
+    // TODO: implement createAnalysisDriver
+    return null;
+  }
+
+  @override
+  void sendNotificationsForSubscriptions(
+      Map<String, List<AnalysisService>> subscriptions) {
+    // TODO: implement sendNotificationsForSubscriptions
+  }
+}
+```