docs: Add uproot4 writing speedup to v0.6.3 release notes (#1601)

* Add large speedup of workspace conversion from JSON to XML + ROOT with uproot v4.1.1 to pyhf v0.6.3 release notes
   - c.f. scikit-hep/uproot5#426

docs/release-notes/v0.6.3.rst

@@ -10,6 +10,9 @@ Important Notes
   ``xmlio`` extra no longer requires ``uproot3`` and all dependencies on
   ``uproot3`` and ``uproot3-methods`` have been dropped.
   (PR :pr:`1567`)
+  ``uproot4`` additionally brings large speedups to writing, which results in an
+  order of magnitude faster conversion time for most workspace conversions from
+  JSON back to XML + ROOT with ``pyhf json2xml``.
 * All backends are now fully compatible and tested with
   `Python 3.9 <https://www.python.org/dev/peps/pep-0596/>`_.
   (PR :pr:`1574`)