docs(changelog): update for 0.9.1

NOTE: this is based on the output of
  git-chglog --next-tag v0.9.1

Author: tueda

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+<a name="0.9.1"></a>
+## [0.9.1] (2022-09-23)
+### Fixed
+- Removed a superfluous message printed when `POSTPROCESS` is not given
+  ([#39](https://github.com/tueda/makefile4latex/issues/39)).
+
+
 <a name="0.9.0"></a>
 ## [0.9.0] (2022-04-22)
 ### Added
@@ -179,6 +186,7 @@
   ```
 
 
+[0.9.1]: https://github.com/tueda/makefile4latex/compare/v0.9.0...v0.9.1
 [0.9.0]: https://github.com/tueda/makefile4latex/compare/v0.8.0...v0.9.0
 [0.8.0]: https://github.com/tueda/makefile4latex/compare/v0.7.0...v0.8.0
 [0.7.0]: https://github.com/tueda/makefile4latex/compare/v0.6.0...v0.7.0