add note to "differences between versions" chapter

book/source/17-versions.md

@@ -5,3 +5,10 @@ SPDX-License-Identifier: CC-BY-SA-4.0
 
 (versions)=
 # Differences between OpenPGP versions
+
+```{admonition} TODO
+:class: warning
+
+write about the delta between v4 and v6, most of all
+- a starting point? https://www.ietf.org/archive/id/draft-ietf-openpgp-crypto-refresh-12.html#name-upgrade-guidance-adapting-i
+```