update docs

Author: KuangYu

README.md

@@ -28,6 +28,7 @@ DOI: 10.1021/acs.jctc.2c01297`
   + [Mbar Estimator](docs/user_guide/4.6MBAR.md)
   + [OpenMM Plugin](docs/user_guide/4.7OpenMMplugin.md)
   + [DiffTraj](docs/user_guide/4.8DiffTraj.md)
+  + [ASE MD interface](docs/user_guide/4.9ASE.md)
 + [5. Advanced examples](docs/user_guide/DMFF_example.ipynb)
 + [And here is a tutorial notebook of the basic usage of DMFF. Welcome to read it and get started with DMFF!](docs/user_guide/test.ipynb)