add Christoph Schmatzler to contributors

Author: izelnakri

README.md

@@ -509,6 +509,7 @@ Many thanks to:
 - [Kian Meng Ang](https://github.com/kianmeng) - Improved documentation
 - [Francisco Correia](https://github.com/fv316) - Made PaperTrail transaction keys and ecto transactions more customizable
 - [Don Barlow](https://github.com/ottobar) - Made :initial_version_key configurable for `strict_mode` inserts
+- [Christoph Schmatzler](https://github.com/cschmatzler) - Built PaperTrail.insert_or_update feature
 - [Izel Nakri](https://github.com/izelnakri) - The Originator of this library. See what I did there ;)
 
 Additional thanks to: