Add book cover and first explanation of the book to the README

Author: spier

README.md

@@ -10,11 +10,17 @@ You are already using InnerSource in your company and want to share your ideas a
 
 ## Mission Statement
 
+<img align="right" src="book/innersource-patterns-book-cover.jpg" width="50%">
+
 Our mission in this working group
 
 - Collect and document agreed upon best practices of how to do InnerSource - in the form of patterns
 - Continuously publish the most mature patterns as an ebook
 
+## The book
+
+We publish the most mature InnerSource Patterns at [innersourcecommons.gitbook.io/innersource-patterns](https://innersourcecommons.gitbook.io/innersource-patterns/). With "most mature" in this context we mean level 2+3 patterns, as shown below.
+
 ## List of Patterns
 
 The below lists all known patterns. They are grouped into three [maturity levels](https://github.com/InnerSourceCommons/InnerSourcePatterns/blob/master/meta/contributor-handbook.md#maturity-levels).