update release notes with cloudpickle

Author: Spaak

RELEASE-NOTES.md

@@ -26,6 +26,7 @@
 - Logp method of `Uniform` and `DiscreteUniform` no longer depends on `pymc3.distributions.dist_math.bound` for proper evaluation (see [#4541](https://github.com/pymc-devs/pymc3/pull/4541)).
 - `Model.RV_dims` and `Model.coords` are now read-only properties. To modify the `coords` dictionary use `Model.add_coord`. Also `dims` or coordinate values that are `None` will be auto-completed (see [#4625](https://github.com/pymc-devs/pymc3/pull/4625)).
 - The length of `dims` in the model is now tracked symbolically through `Model.dim_lengths` (see [#4625](https://github.com/pymc-devs/pymc3/pull/4625)).
+- We now include `cloudpickle` as a required dependency, and no longer depend on `dill` (see [#4858](https://github.com/pymc-devs/pymc3/pull/4858)).
 - ...
 
 ## PyMC3 3.11.2 (14 March 2021)