add release note

Author: aloctavodia

RELEASE-NOTES.md

@@ -18,6 +18,7 @@
 - SMC: stabilize covariance matrix [3573](https://github.com/pymc-devs/pymc3/pull/3573)
 - SMC is no longer a step method of `pm.sample` now it should be called using `pm.sample_smc` [3579](https://github.com/pymc-devs/pymc3/pull/3579)
 - SMC: improve computation of the proposal scaling factor [3594](https://github.com/pymc-devs/pymc3/pull/3594)
+- SMC: reduce number of logp evaluations [3600](https://github.com/pymc-devs/pymc3/pull/3600)
 - Now uses `multiprocessong` rather than `psutil` to count CPUs, which results in reliable core counts on Chromebooks.
 - `sample_posterior_predictive` now preallocates the memory required for its output to improve memory usage. Addresses problems raised in this [discourse thread](https://discourse.pymc.io/t/memory-error-with-posterior-predictive-sample/2891/4).
 - Fixed a bug in `Categorical.logp`. In the case of multidimensional `p`'s, the indexing was done wrong leading to incorrectly shaped tensors that consumed `O(n**2)` memory instead of `O(n)`. This fixes issue [#3535](https://github.com/pymc-devs/pymc3/issues/3535)