README.md in activefedora-aggregation-0.3.0 vs README.md in activefedora-aggregation-0.4.0

@@ -1,10 +1,15 @@
 [![Gem Version](https://badge.fury.io/rb/activefedora-aggregation.svg)](http://badge.fury.io/rb/activefedora-aggregation) [![Build Status](https://circleci.com/gh/projecthydra-labs/activefedora-aggregation.svg?style=shield&circle-token=:circle-token)](https://circleci.com/gh/projecthydra-labs/activefedora-aggregation)
 # ActiveFedora::Aggregation
 
-Aggregations for ActiveFedora.
+Aggregations for ActiveFedora: manage a group of related objects using predicates from the
+[OAI-ORE data model](http://www.openarchives.org/ore/1.0/datamodel).  Provides the foundation
+for flexible relationships, including items appearing multiple times in a group,
+flexible/optional ordering, etc.
 
+Used extensively by [Hydra::PCDM](https://github.com/projecthydra-labs/hydra-pcdm/).
+
 ### Example
 ```ruby
 class GenericFile < ActiveFedora::Base
 end
 
@@ -19,6 +24,6 @@
 image.generic_files = [generic_file2, generic_file1]
 image.save
 
 ```
 
-Now the `generic\_files` method returns an ordered list of GenericFile objects.
+Now the `generic_files` method returns an ordered list of GenericFile objects.