Add more documentation for plugins

pkg/analyzer_plugin/doc/tutorial/navigation.md

+# Providing Navigation
+
+Navigation information is used by clients to allow users to navigate to the
+location at which an identifier is defined.
+
+## Implementation details
+
+Navigation information can be requested both by an `analysis.getNavigation`
+request and by a subscription. If the server has subscribed for navigation
+information in some set of files, the the plugin should send the information in
+an `analysis.navigation` notification whenever the information needs to be
+updated.
+
+When an `analysis.getNavigation` request is received, the method
+`handleAnalysisGetNavigation` will be invoked. This method is responsible for
+returning a response that contains the available navigation information.
+
+The easiest way to implement the method `handleAnalysisGetNavigation` is by
+adding the classes `NavigationMixin` and `DartNavigationMixin` (from
+`package:analyzer_plugin/plugin/navigation_mixin.dart`) to the list of mixins
+for your subclass of `ServerPlugin`. This will leave you with one abstract
+method that you need to implement: `getNavigationContributors`. That method is
+responsible for returning a list of `NavigationContributor`s. It is the
+navigation contributors that produce the actual navigation information. (Most
+plugins will only need a single navigation contributor.)
+
+To write a navigation contributor, create a class that implements
+`NavigationContributor`. The interface defines a single method named
+`computeNavigation`. The method has two arguments: a `NavigationRequest` that
+describes the region of the file for which navigation is being requested and a
+`NavigationCollector` through which navigation information is to be added.
+
+## Example
+
+Start by creating a class that implements `NavigationContributor`, then
+implement the method `computeNavigation`. This method is typically implemented
+by creating a visitor (such as an AstVisitor) that can visit the results of the
+analysis (such as a CompilationUnit) and extract the navigation information from
+the analysis result.
+
+For example, your contributor might look something like the following:
+
+```dart
+class MyNavigationContributor implements NavigationContributor {
+  @override
+  void computeNavigation(
+      NavigationRequest request, NavigationCollector collector) {
+    if (request is DartNavigationRequest) {
+      NavigationVisitor visitor = new NavigationVisitor(collector);
+      request.result.unit.accept(visitor);
+    }
+  }
+}
+
+class NavigationVisitor extends RecursiveAstVisitor {
+  final NavigationCollector collector;
+
+  NavigationVisitor(this.collector);
+
+  @override
+  void visitSimpleIdentifier(SimpleIdentifier node) {
+    // ...
+  }
+}
+```
+
+Given a contributor like the one above, you can implement your plugin similar to
+the following:
+
+```dart
+class MyPlugin extends ServerPlugin with NavigationMixin, DartNavigationMixin {
+  // ...
+
+  @override
+  List<NavigationContributor> getNavigationContributors(String path) {
+    return <NavigationContributor>[new MyNavigationContributor()];
+  }
+}
+```