update release notes

doc/release_notes.rst

@@ -1,8 +1,12 @@
 Release Notes
 =============
 
-Upcoming Version
-----------------
+.. Upcoming Version
+.. ----------------
+
+
+Version 0.3.6
+-------------
 
 * The handling of `pandas` objects was improved. As `pandas` objects are fully aware of coordinates, their index and columns are now strictly taken into account. For example, when multiplying a `pandas.DataFrame` with a variable, linopy now checks the alignment of indexes and reindexes accordingly. Previously, if the axis shapes were the same, the indexes of the variable were inserted and the `pandas` indexes were effectively ignored. A warning has been added for cases where users should expect changes to the results with this version. **Important**: This does not apply to overwriting the coordinates when one expression is added to another, e.g. "x + df" still overwrites the index of "df" when the dimensional shapes are aligned.
 * The `.mask` attribute of the `Constraint` class was fixed to return a proper boolean `xarray.DataArray` object.