Add to deprecated section of whats new

Author: jsignell

doc/whats-new.rst

@@ -32,6 +32,15 @@ Breaking changes
 Deprecations
 ~~~~~~~~~~~~
 
+- Start deprecation cycle for changing the default keyword arguments to ``concat``, ``merge``, ``combine``, ``open_mfdataset``.
+  Emits a ``FutureWarning`` when using old defaults and new defaults would result in different behavior.
+  Adds an option: ``use_new_combine_kwarg_defaults`` to opt in to new defaults immediately.
+  New values are:
+    - ``data_vars``: "minimal"
+    - ``coords``: "minimal"
+    - ``compat``: "override"
+    - ``join``: "exact"
+  By `Julia Signell <https://github.com/jsignell>`_.
 
 Bug fixes
 ~~~~~~~~~