update changelog (I hope I'm doing this right)

Author: guillaumechauvat

CHANGELOG.md

@@ -27,6 +27,11 @@ Security in case of vulnerabilities.
 
 ## [Unreleased]
 
+## [1.3.1] - 2020-09-17
+
+### Changed
+- use ndarray.tofile() for faster output 
+
 ## [1.3.0.post0] - 2020-07-16
 
 ### Fixed
@@ -50,7 +55,8 @@ Security in case of vulnerabilities.
 ### Changed
 - Miscellaneous improvements in documentation, testing and packaging
 
-[Unreleased]: https://github.com/jcanton/pymech/compare/1.3.0.post0...HEAD
+[Unreleased]: https://github.com/jcanton/pymech/compare/1.3.1...HEAD
+[1.3.1]: https://github.com/jcanton/pymech/compare/1.3.0.post0...1.3.1
 [1.3.0.post0]: https://github.com/jcanton/pymech/compare/1.3.0...1.3.0.post0
 [1.3.0]: https://github.com/jcanton/pymech/compare/1.2.0...1.3.0
 [1.2.0]: https://github.com/jcanton/pymech/releases/tag/1.2.0