Update whats_new and citation for 1.8.4 (#467)

Author: aaronreidsmith

CITATION.cff

@@ -13,8 +13,8 @@ keywords:
   - "forecasting models"
   - sarimax
 title: "pmdarima"
-version: 1.8.3
-date-released: 2021-09-24
+version: 1.8.4
+date-released: 2021-11-05
 license: MIT
 repository-artifact: https://pypi.org/project/pmdarima
 repository-code: https://github.com/alkaline-ml/pmdarima

doc/whats_new.rst

@@ -7,6 +7,11 @@ What's new in pmdarima
 As new releases of pmdarima are pushed out, the following list (introduced in
 v0.8.1) will document the latest features.
 
+`v1.8.4 <http://alkaline-ml.com/pmdarima/1.8.4/>`_
+--------------------------------------------------
+
+* Add compatibility for ``statsmodels`` 0.13 and higher
+
 `v1.8.3 <http://alkaline-ml.com/pmdarima/1.8.3/>`_
 --------------------------------------------------