add documentation for cube

docs/cube/README.md

@@ -1,6 +1,6 @@
 # Manipulate CP2K Cube Files
 
-The `CP2KData` Python package provides tools for working with cube files generated by the CP2K quantum chemistry software. One of the standout features of this package is its ability to handle CP2K cube files and perform various analyses.
+The `CP2KData` Python package provides tools for working with cube files generated by the CP2K software. One of the features of this package is its ability to handle CP2K cube files and perform various analyses.
 
 ## Getting Started
 Import the necessary modules and load a cube file:
@@ -35,7 +35,7 @@ ase.atoms.Atoms
 ```
 
 ## Integration over space
-User can obtain the integration over space using the `get_integration()` method, for example, if you get integration using density cube, you will get the total number electrons in this cube
+User can obtain the integration over the full unit cell using the `get_integration()` method, for example, if you get integration using density cube, you will get the total number electrons in this cube
 ```python
 mycube.get_integration()
 ```
@@ -45,6 +45,21 @@ mycube.get_integration()
 ```
 The result is not exactly an integer. User should round it to integer manually.
 
+Sometimes, we need to integrate over a certain area instead of the full unit cell.
+One can use `start_x`, `end_x`, `start_y`, `end_y`, `start_z`, and `end_z` variables to define the desired range of values for the x, y, and z axes.
+```python
+mycube = Cp2kCube("example.cube")
+print(mycube.get_integration(start_z = 41.76))
+print(mycube.get_integration(end_z = 41.76))
+mycube.get_integration()
+```
+```shell
+# output
+0.8702837077294211
+0.12970844915317598
+```
+Note that the integration by area is not applied to non-orthogonal cells.
+
 ## Planar Averaging
 You can calculate planar average data from the cube file, both with and without interpolation:
 ```python