changelog.md in natural_number_string-0.3.0 vs changelog.md in natural_number_string-0.3.1

@@ -1,4 +1,10 @@
+<a name="0.3.1"></a>
+## [0.3.1](https://github.com/packsaddle/ruby-natural_number_string/compare/v0.3.0...v0.3.1) (2015-09-26)
+
+Update document.
+
+
 <a name="0.3.0"></a>
 # [0.3.0](https://github.com/packsaddle/ruby-natural_number_string/compare/v0.2.1...v0.3.0) (2015-09-26)
 
 Drop `#natural_number_string?`.
 Use `#positive_integer_string?` or `zero_or_positive_integer_string?`.