Update README.md

README.md

@@ -3,4 +3,21 @@ X-ray diffraction calculations
 This branch is initally planned for my Computational Physics class in 2017 fall.
 https://github.com/qzhu2017/2017-cmp
 
-It might be expanded in near feature.
+Currently, there are three classes,
+- Element
+- crystal
+- XRD
+
+One could load the crystal from 
+- dictionary
+- POSCAR
+- CIF file (to add in near future)
+
+To perform XRD calculation, one needs to provide the following info
+- crystal structure
+- wavelength (default is Cu-Ka: 1.54184 \AA)
+- maximum 2\theta value (defult: 180 degree)
+
+The atomic scattering factor is calculated from 9-parameter equation by Don Cromer and J. Mann.
+
+More detailed usage could be found in the jupyter notebook.