Add changelog

ChangeLog.md

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+
+# Changelog
+
+## [0.3.0]
+
+### Added
+
+- It is now possible to add an instantiated instance of CommandOptions with `add_command`.  This
+  makes it easier to access global options from within command options.  See the test
+  shouldAccessParentOptionsFromCommand in `command_t.cpp` and the new example in the README file.
+- The help formatter can be changed in parser configuration. See the test shouldChangeHelpFormatter
+  in `parserconfig_t.cpp`.
+
+### Changed
+
+- The interface for defining program parameters has changed.  The parameters are defined through the
+  `params()` method of the `argument_parser` class.  The old `argument_parser` methods for defining
+  parameters are deprecated and will be removed.
+- Fix: Treat negative numbers as numbers when they can not be mistaken for options.
+