The Apache Sling CMS is a reference implementation of a fully-featured Content Management System built in Apache Sling. This application showcases multiple Apache Sling features such as Context Aware Configurations, Resource Mapping, Rewriter and Resource Merging.
Interested in contributing? Here's some great places to get started:
You will find all the necessary details about how you can do this at https://sling.apache.org/contributing.html.