Adding paper to the Readme

[zaccharieramzi]

We should have the Pysap paper linked in the readme with an associated bibtex citation in order for people to more easily reference this work.

[philouc]

Absolutely ; here is the bibtex reference :
@Article{farrens2020pysap,
title={{PySAP: Python Sparse Data Analysis Package for multidisciplinary image processing}},
author={Farrens, S and Grigis, A and El Gueddari, L and Ramzi, Z and Chaithya, GR and Starck, S and Sarthou, B and Cherkaoui, H and Ciuciu, P and Starck, J-L},
journal={Astronomy and Computing},
volume={32},
pages={100402},
year={2020},
publisher={Elsevier}
}