docs(CHANGES): Note deprecation of which()

Author: tony

CHANGES

@@ -14,6 +14,8 @@ $ pip install --user --upgrade --pre libtmux
 
 ### Breaking changes
 
+- Remove `common.which()` in favor of {func}`shutil.which`, Credit:
+  @rocksandska, via #407
 - Fixes #402: {func}`common.tmux_cmd` will only strip _trailing_ empty lines. Before this change,
   all empty lines were filtered out. This will lead to a more accurate behavior when using
   {meth}`Pane.capture_pane`. Credit: @rockandska, via #405.