README: Document requirement of sdt-python >= 15.2

Author: Lukas Schrangl

README.md

@@ -26,7 +26,7 @@ analyze single molecule FRET data.
 
 - Python >= 3.6
 - Jupyter Notebook or Lab
-- sdt-python >= 14.4
+- sdt-python >= 15.2
 - numpy
 - scipy
 - pandas