Add comment to RELEASE-NOTES.md

Brandon T. Willard · michaelosthege · commit 9218f336994b · 2021-03-10T15:48:24.000+01:00
diff --git a/RELEASE-NOTES.md b/RELEASE-NOTES.md
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 ### Maintenance
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+- ⚠ Our memoization mechanism wasn't robust against hash collisions (#4506), sometimes resulting in incorrect values in, for example, posterior predictives. The `pymc3.memoize` module was removed and replaced with `cachetools`.  The `hashable` function and `WithMemoization` class were moved to `pymc3.util`.
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 ## PyMC3 3.11.1 (12 February 2021)
 
