Mention tmm

Author: MarJMue

README.md

@@ -33,4 +33,5 @@ python setup.py install
 ```
 
 ## Acknowledgements
-- Based on Olivier Castany's [Berreman4x4](https://github.com/Berreman4x4/Berreman4x4)
+- Based on Olivier Castany's [Berreman4x4](https://github.com/Berreman4x4/Berreman4x4)
+- Solver2x2 based on Steve Byrnes's [tmm](https://github.com/sbyrnes321/tmm)