improve README

At the top of the README.md add more contextual info about the reasoning for creating this project, to store parsivel data into netCDF, in a self-documented manner.

We can also 1 or two images.

A good example of this can be found at https://github.com/actris-cloudnet/mwrpy/tree/main#readme

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information