Add Changelog entry.

CHANGELOG.md

@@ -90,7 +90,9 @@ sed 's/\bPersistentP\b/Persistent/g; s/\bTimelessP\b/Timeless/g; s/\bCMRADiscret
   + The version of big operations over lists was redefined so that it enjoys
     more definitional equalities.
 * Various improvements to `solve_ndisj`.
-* Improve handling of pure (non-state-dependent) reductions in heap_lang.
+* Some extensions/improvements of heap_lang:
+  - Improve handling of pure (non-state-dependent) reductions.
+  - Add fetch-and-add (`FAA`) operation.
 * Use `Hint Mode` to prevent Coq from making arbitrary guesses in the presence
   of evars, which often led to divergence. There are a few places where type
   annotations are now needed.