Update changelog

ChangeLog.rst

+7.0.3 - TBD
+-----------
+
+* Worked around broken ``numeric_limits`` for 128-bit integers
+  (`#1787 <https://github.com/fmtlib/fmt/issues/1787>`_).
+
+* Added error reporting on missing named arguments
+  (`#1796 <https://github.com/fmtlib/fmt/issues/1796>`_).
+
+* Stopped using 128-bit integers with clang-cl
+  (`#1800 <https://github.com/fmtlib/fmt/pull/1800>`_).
+  Thanks `@Kingcom <https://github.com/Kingcom>`_.
+
+* Fixed issues in locale-specific integer formatting (#)
+  (`#1782 <https://github.com/fmtlib/fmt/issues/1782>`_,
+  `#1801 <https://github.com/fmtlib/fmt/issues/1801>`_)
+
 7.0.2 - 2020-07-29
 ------------------