CHANGELOG entry.

CHANGELOG.md

@@ -8,6 +8,8 @@ API-breaking change is listed.
   `difference_difference_r` and `difference_difference_r_L`.
 - Let `set_solver` use `eauto` (instead of `idtac`) as its default solver.
 - Add tactic `tc_solve` (this was `iSolveTC` in Iris).
+- Let `f_equiv` to use `reflexivity` in a way similar to `f_equal`. That is,
+  let `f_equiv` solve goals and subgoals of the form `R x x`.
 
 The following `sed` script should perform most of the renaming
 (on macOS, replace `sed` by `gsed`, installed via e.g. `brew install gnu-sed`).