Explain a bit better which patterns are published in the book.

Author: spier

book/introduction.md

@@ -12,7 +12,7 @@ Welcome to the **InnerSource Patterns book**.
 
 This book contains InnerSource best practices codified in a specific format to make it easy to understand, evaluate, and apply them in your context. We call this format a **pattern**.
 
-These patterns have been collected by the [InnerSource Commons](http://innersourcecommons.org) community over many years. The most mature of these patterns have been published in this book.
+These patterns have been collected by the [InnerSource Commons](http://innersourcecommons.org) community over many years. The most mature patterns have been published in this book. Mature in this context means that each pattern has been reviewed by members of the community, and has at least one known instance where this pattern has been used.
 
 In this introduction we explain [what InnerSource is](#what-is-innersource), [what a pattern is](#what-are-innersource-patterns), and [how to use these patterns](#how-can-you-use-innersource-patterns) in your organization.