Added library-level comments for polymer and polymer_expressions

pkg/polymer_expressions/README.md

 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:
 
 ```html
     {{ 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
 data are automatically reflected in the DOM, and changes in HTML inputs are
 assigned back into the model. The template and model are bound together via
 binding expressions that are evaluated against the model. These binding
 expressions are placed in double-curly-braces, or "mustaches".
 
 Example:
 
@@ skipping 248 matching lines @@
 [web-ui@dartlang.org mailing list][web-ui-list].
 
 Please [file issues on Dart project page](http://dartbug.com/new)
 for any bugs you find or for feature requests. Make a note that it applies to
 "package:polymer_expressions"
 
 You can discuss Polymer Expressions on the
 [web-ui@dartlang.org mailing list][web-ui-list].
 
 [web-ui-list]: https://groups.google.com/a/dartlang.org/forum/#!forum/web-ui