[REQUEST] Add citation and link to the CELLxGENE preprint to the README

[Sara Simmonds]

Please cite our preprint if you use CZ CELLxGENE in your work.
Citation:
cellxgene: a performant, scalable exploration platform for high dimensional sparse matrices
Colin Megill, Bruce Martin, Charlotte Weaver, Sidney Bell, Lia Prins, Seve Badajoz, Brian McCandless, Angela Oliveira Pisco, Marcus Kinsella, Fiona Griffin, Justin Kiggins, Genevieve Haliburton, Arathi Mani, Matthew Weiden, Madison Dunitz, Maximilian Lombardo, Timmy Huang, Trent Smith, Signe Chambers, Jeremy Freeman, Jonah Cool, Ambrose Carr
bioRxiv 2021.04.05.438318; doi: https://doi.org/10.1101/2021.04.05.438318