Add release note

Author: ricardoV94

RELEASE-NOTES.md

@@ -14,6 +14,7 @@
 - `ScalarSharedVariable` can now be used as an input to other RVs directly (see [#4445](https://github.com/pymc-devs/pymc3/pull/4445)).
 - `pm.sample` and `pm.find_MAP` no longer change the `start` argument (see [#4458](https://github.com/pymc-devs/pymc3/pull/4458)).
 - Fixed `Dirichlet.logp` method to work with unit batch or event shapes (see [#4454](https://github.com/pymc-devs/pymc3/pull/4454)).
+- Bugfix in logp and logcdf methods of `Triangular` distribution (see[#4470](https://github.com/pymc-devs/pymc3/pull/4470)).
 
 ## PyMC3 3.11.0 (21 January 2021)