diff --git a/book/source/01-intro.md b/book/source/01-intro.md
index 22904f0..e355734 100644
--- a/book/source/01-intro.md
+++ b/book/source/01-intro.md
@@ -21,7 +21,11 @@ Three groups of people interact with OpenPGP:
 
 This document is focused on the second group, software developers, who use OpenPGP functionality in their software projects. It describes the properties of the OpenPGP system and its uses. It presupposes solid knowledge of software development concepts and of general cryptographic concepts. Thus, this text describes OpenPGP at the "library-level," teaching concepts that will help software developers get started as a user of any implementation (e.g., OpenPGP JS, Sequoia PGP).
 
-```Heiko, we should elaborate a bit on why here```
+```{admonition} TODO
+:class: warning
+
+Heiko, we should elaborate a bit on why here
+```
 
 This document is not intended for end-users. It is also not for implementers of OpenPGP libraries (or other software that directly handles internal OpenPGP data structures).