CHANGELOG entry.

CHANGELOG.md

@@ -42,6 +42,8 @@ Coq development, but not every API-breaking change is listed.  Changes marked
 * Removed `Core` type class for defining the total core; it is now always
   defined in terms of the partial core. The only user of this type class was the
   STS RA.
+* Added the type `siProp` of "plain" step-indexed propositions, together with
+  basic proofmode support.
 
 ## Iris 3.2.0 (released 2019-08-29)