Add documentation

[goldrafa]

Nice project! However, the documentation seems quite incomplete. Apart from a general description of the project and the pip install procedure, there is no further documentation on usage and code implementation. Even though there is nbdev-markup throughout the code, the Jupyter notebooks found in the file structure include only the testing of the individual classes and functions. Last but not least, there are no comments regarding the use of pytest for the feems package and nbdev_test on the other two packages contained in this project.

Happy for any clarification, if I'm missing the point here.