Added release note

Author: TomAugspurger

doc/source/whatsnew/v0.21.0.txt

@@ -560,6 +560,8 @@ Other API Changes
 - :func:`to_datetime` when passed a tz-aware ``origin=`` kwarg will now raise a more informative ``ValueError`` rather than a ``TypeError`` (:issue:`16842`)
 - Renamed non-functional ``index`` to ``index_col`` in :func:`read_stata` to improve API consistency (:issue:`16342`)
 - Bug in :func:`DataFrame.drop` caused boolean labels ``False`` and ``True`` to be treated as labels 0 and 1 respectively when dropping indices from a numeric index. This will now raise a ValueError (:issue:`16877`)
+- Pandas no longer registers matplotlib converters on import. The converters
+  will be registered and used when the first plot is draw (:issue:`17710`)
 
 .. _whatsnew_0210.deprecations: