Initial documentation for the plugin package

pkg/analyzer_plugin/README.md

Index: pkg/analyzer_plugin/README.md
diff --git a/pkg/analyzer_plugin/README.md b/pkg/analyzer_plugin/README.md
index 954aa0ef26ae5ad4369ad380b6e202693870c803..42cf20a74b34129925b5122b3e1df85d8af53ad7 100644
--- a/pkg/analyzer_plugin/README.md
+++ b/pkg/analyzer_plugin/README.md
@@ -4,6 +4,18 @@ A framework for building plugins for the analysis server.
 
 ## Usage
 
+**Note:** The plugin support is not currently available for general use.
+
+Plugins are written in Dart and are run in the same VM as the analysis server.
+The analysis server runs each plugin in a separate isolate and communicates with
+the plugin using a [plugin API][pluginapi]. This API is similar to the API used
+by the analysis server to communicate with clients.
+
+Plugins are automatically discovered and run by the analysis server.
+
+This package contains support code to make it easier to write a plugin. There is
+a [tutorial][tutorial] describing how to use the support in this package.
+
 ## Support
 
 Post issues and feature requests on the [issue tracker][issues].
@@ -19,3 +31,4 @@ See the [LICENSE] file.
 [LICENSE]: https://github.com/dart-lang/sdk/blob/master/pkg/analyzer/LICENSE
 [list]: https://groups.google.com/a/dartlang.org/forum/#!forum/analyzer-discuss
 [pluginapi]: https://htmlpreview.github.io/?https://github.com/dart-lang/sdk/blob/master/pkg/analyzer_plugin/doc/api.html
+[tutorial]: doc/tutorial/tutorial.md