add a line to RELEASE-NOTES.md

Author: tohtsky

RELEASE-NOTES.md

@@ -9,6 +9,8 @@ This is the first release to support Python3.9 and to drop Python3.6.
 - Make `sample_shape` same across all contexts in `draw_values` (see [#4305](https://github.com/pymc-devs/pymc3/pull/4305)).
 - Removed `theanof.set_theano_config` because it illegally touched Theano's privates (see [#4329](https://github.com/pymc-devs/pymc3/pull/4329)).
 
+### New Features
+- `OrderedProbit` distribution added (see [#4232](https://github.com/pymc-devs/pymc3/pull/4232)).
 
 ## PyMC3 3.10.0 (7 December 2020)