Add more documentation for plugins

pkg/analyzer_plugin/doc/tutorial/tutorial.md

Index: pkg/analyzer_plugin/doc/tutorial/tutorial.md
diff --git a/pkg/analyzer_plugin/doc/tutorial/tutorial.md b/pkg/analyzer_plugin/doc/tutorial/tutorial.md
index 255137fbd8ac2b37761de986e2d849f0d7c4c576..93db766434246769fa8e2071e917e1ea129c1c85 100644
--- a/pkg/analyzer_plugin/doc/tutorial/tutorial.md
+++ b/pkg/analyzer_plugin/doc/tutorial/tutorial.md
@@ -1,13 +1,17 @@
 # Building a Plugin
 
-This is the introduction page to a set of pages that describe how to implement a
-plugin. You should probably read the [Getting Started][gettingStarted] page
-first, but there is no specific order for the remaining pages.
+This is the table of contents for a set of pages that describe how to implement
+a plugin. You should probably read the [Introduction][introduction] and
+[Getting Started][gettingStarted] pages first, but the remaining pages can be
+read as you attempt to implement the described functionality.
 
 ## Pages
 
 The following is a list of the pages available in this tutorial.
 
+[Introduction][introduction] -
+What is a plugin and what can it do?
+
 [Getting Started][gettingStarted] -
 How to write a minimal plugin.
 
@@ -28,3 +32,4 @@ How to provide code completion suggestions.
 [creatingEdits]: creating_edits.md
 [fixes]: fixes.md
 [gettingStarted]: getting_started.md
+[introduction]: introduction.md