Prepare for v0.5.0

Author: malmans2

doc/whats-new.rst

@@ -3,7 +3,7 @@
 What's New
 ----------
 
-v0.4.1 (unreleased)
+v0.5.0 (Feb 24, 2021)
 ===================
 
 - Replace ``cf.describe()`` with :py:meth:`Dataset.cf.__repr__`. By `Mattia Almansi`_.