Document user-visible changes

doc/user-changes.md

@@ -4,6 +4,15 @@ This document is a development diary summarizing changes in `alr` that notably
 affect the user experience. It is intended as a one-stop point for users to
 stay on top of `alr` new features.
 
+### Document caret/tilde use for pre-1.0 versions, and warn about it
+
+PR [#669](https://github.com/alire-project/alire/pull/669).
+
+Alire does not change the meaning of caret (^) and tilde (~) operators for
+pre/post-1.0 versions. This interpretation has been clarified in the catalog
+specification, and `alr` will warn about any suspicious usage. This warning may
+be disabled by the user with the new `warning.caret` configuration option.
+
 ### Switch to check for unknown enumeration values in the index
 
 PR [#656](https://github.com/alire-project/alire/pull/656).