DOC: Update change log

Update for recent changes

Author: bashtage

doc/source/changes/4.0.txt

@@ -1,5 +1,9 @@
 Changes since 4.0
 =================
+- Enable user to specify a specific value of the `backcast` in place of
+  the automatically generated value.
+- Fixed a big where parameter-less models where incorrectly reported as
+  having constant variance (:issue:`248`)
 - Added support for MIDAS volatility processes using Hyperbolic
   weighting (:issue:`233`)
 - Added a parameter to forecast that allows a user-provided callable random