[skip-ci][python] Update release notes to include UHI

Author: siliataider

Diff for: README/ReleaseNotes/v636/index.md

@@ -29,6 +29,7 @@ The following people have contributed to this new version:
  Manuel Tobias Schiller, University of Glasgow,\
  Surya Somayyajula, UMass Amherst,\
  Petr Stepanov, @petrstepanov,\
+ Silia Taider, CERN/EP-SFT,\
  Dongliang Zhang, University of Science and Technology of China
 
 ## Deprecation and Removal
@@ -46,6 +47,14 @@ The following people have contributed to this new version:
 
 ## Python Interface
 
+### UHI
+* ROOT histograms now comply with the [Unified Histogram Interface (UHI)](https://uhi.readthedocs.io/en/latest/index.html) specification, enhancing interoperability with other UHI-compatible libraries and standardizing histogram operations.
+  The following features were added:
+  * Implemented the UHI `PlottableHistogram` protocol enabling ROOT histograms to be plotted by any library supporting `PlottableHistogram` objects.
+  * Introduced UHI-style indexing for access and setting bin values.
+  * Introduced UHI-style slicing for selecting histogram ranges.
+  * Implemented the `ROOT.uhi.loc`, `ROOT.uhi.underflow`, `ROOT.uhi.overflow`, `ROOT.uhi.rebin`, and `ROOT.uhi.sum` tags.
+
 ## RDataFrame
 - When running multiple computation graphs run concurrently using [`RunGraphs()`](https://root.cern/doc/master/namespaceROOT_1_1RDF.html#a526d77d018bf69462d736bbdd1a695c4),
   the pool of slot numbers that a thread can pick from is now shared across all graphs. This enables use cases where a single resource, which may be expensive to create or copy,