ch4: add todo

book/source/04-certificates.md

@@ -135,6 +135,11 @@ Instead, key flags, along with other metadata about that component key, such as
 - For the primary key, its key flags and other metadata can be defined in two ways: they can be linked with the [Primary User ID](primary_user_id) or through a [direct key signature](direct_key_signature).
 - For subkeys, the key flags and other metadata are set using the mechanism that binds the subkey to the certificate, specifically through the primary key. Further details on  [binding subkeys](binding_subkeys) are below.
 
+```{admonition} TODO
+:class: warning
+
+Write a section about algorithm preference/feature signaling
+```
 
 (identity_components)=
 ## Identity components