Add Addendum to the about page

Author: diehlpk

about.md

@@ -8,4 +8,6 @@ Webpage of the [Book](https://link.springer.com/book/10.1007/978-3-031-54369-2)
 
 The examples are available on [GitHub](https://github.com/ModernCPPBook/Examples) and the C++ Explorer is available on [GitHub](https://github.com/stevenrbrandt/CxxExplorer).
 
+As carefully we proofread the book, there will be always some bugs or issues which we will collect in the [Addendum](https://hpxbook.stellar-group.org/addendum/2024/08/15/addendum.html). 
+
 This blog will showcase new features of the C++ 23 and C++ 26 standard mentioned in the book.