Add changelog

Author: satoryu

CHANGELOG.md

@@ -0,0 +1,13 @@
+# Changelog
+
+## v0.4.0
+
+### Enhancements
+
+- Implement substitute method on TextRun class. [#75](https://github.com/ruby-docx/docx/pull/75)
+
+### Improvements
+
+- Updates dependencies. [#72](https://github.com/ruby-docx/docx/pull/72), [#77](https://github.com/ruby-docx/docx/pull/77)
+- Fix: #paragraphs grabs paragraphs in tables. [#76](https://github.com/ruby-docx/docx/pull/76)
+- Updates supported ruby versions. [#78](https://github.com/ruby-docx/docx/pull/78)