Add changelog entry for `PureExec`.

CHANGELOG.md

@@ -79,6 +79,7 @@ Changes in Coq:
 * Changed `IntoVal` to be directly usable for rewriting `e` into `of_val v`, and
   changed `AsVal` to be usable for rewriting via the `[v <-]` destruct pattern.
 * `wp_fork` is now written in curried form.
+* `PureExec`/`wp_pure` now supports taking multiple steps at once.
 
 ## Iris 3.1.0 (released 2017-12-19)