Update RELEASE-NOTES.md

Add Bayesian R2 PR #2696.

Author: Junpeng Lao

RELEASE-NOTES.md

@@ -11,7 +11,7 @@
 - Fixed `compareplot` to use `loo` output. 
 - Add test for `model.logp_array` and `model.bijection` (#2724) 
 - Fixed `sample_ppc` and `sample_ppc_w` to iterate all chains(#2633)
-- Add test for `stats.r2_score` (#2729)
+- Add Bayesian R2 score (for GLMs) `stats.r2_score` (#2696) and test (#2729).