Added library-level comments for polymer and polymer_expressions

pkg/polymer_expressions/README.md

Index: pkg/polymer_expressions/README.md
diff --git a/pkg/polymer_expressions/README.md b/pkg/polymer_expressions/README.md
index 5a7e9b29cbd40e4c163ca5027f93096466c01996..f01b17ce2e6fe10878f70beee57363a2b4393e00 100644
--- a/pkg/polymer_expressions/README.md
+++ b/pkg/polymer_expressions/README.md
@@ -1,8 +1,24 @@
 polymer_expressions
 ===================
 
-Polymer Expressions are an expressive syntax that can be used in templates in
-Dart. Polymer Expressions allow you to write complex binding expressions, with
+
+Polymer expressions are an expressive syntax that can be used in templates in
+Dart.
+
+Templates are one feature of Polymer.dart—a set of comprehensive UI
+and utility components for building web applications.
+This package is automatically included with the
+[polymer](https://pub.dartlang.org/packages/polymer) package
+because polymer expressions are often used in polymer Dart apps.
+The [Polymer.dart home page][home_page]
+contains a list of features, project status,
+installation instructions, instructions for upgrading from Web UI,
+and links to other documentation.
+
+
+## Overview
+
+Polymer Expressions allow you to write complex binding expressions, with
 property access, function invocation, list/map indexing, and two-way filtering
 like:
 
@@ -10,8 +26,6 @@ like:
     {{ person.title + " " + person.getFullName() | upppercase }}
 ```
 
-## Overview
-
 ### Model-Driven Views (MDV)
 [MDV][mdv] allows you to define templates directly in HTML that are rendered by the
 browser into the DOM. Templates are bound to a data model, and changes to the