add pymc-marketing to README

Author: williambdean

README.rst

@@ -254,6 +254,7 @@ Domain specific
 - `Exoplanet <https://github.com/dfm/exoplanet>`__: a toolkit for modeling of transit and/or radial velocity observations of exoplanets and other astronomical time series.
 - `beat <https://github.com/hvasbath/beat>`__: Bayesian Earthquake Analysis Tool.
 - `CausalPy <https://github.com/pymc-labs/CausalPy>`__: A package focussing on causal inference in quasi-experimental settings.
+- `PyMC-Marketing <https://github.com/pymc-labs/pymc-marketing>`__: Bayesian marketing toolbox for marketing mix modeling, customer lifetime value, and more.
 
 Please contact us if your software is not listed here.