Add pre-release install instructions to whats-new (#6740)

Author: dcherian

doc/whats-new.rst

@@ -68,6 +68,14 @@ This pre-release brings a number of bug fixes and improvements, most notably a m
 refactor of the indexing functionality and the use of `flox`_ in ``groupby`` operations. It also stops
 testing support for the abandoned PyNIO.
 
+Install it using
+
+::
+
+    mamba create -n <name> python=3.10 xarray
+    python -m pip install --pre --upgrade --no-deps xarray
+
+
 Many thanks to the 39 contributors:
 
 Abel Soares Siqueira, Alex Santana, Anderson Banihirwe, Benoit Bovy, Blair Bonnett, Brewster