Add citation to readme

Just to make it more obvious (i.e. not just the citation.cff file).

README.md

@@ -49,3 +49,12 @@ See [our blog posts](https://brainglobe.info/blog/) for more information on the
 If you have encountered a bug whilst using cellfinder, please [open an issue on GitHub](https://github.com/brainglobe/cellfinder/issues).
 
 If you are interested in contributing to cellfinder (thank you!) - please head over to our [developer documentation](https://brainglobe.info/community/developers/index.html).
+
+## Citation
+If you find this package useful, and use it in your research, please cite the following paper:
+> Tyson, A. L., Rousseau, C. V., Niedworok, C. J., Keshavarzi, S., Tsitoura, C., Cossell, L., Strom, M. and Margrie, T. W. (2021) “A deep learning algorithm for 3D cell detection in whole mouse brain image datasets’ PLOS Computational Biology, 17(5), e1009074
+[https://doi.org/10.1371/journal.pcbi.1009074](https://doi.org/10.1371/journal.pcbi.1009074)
+
+**If you use this, or any other tools in the brainglobe suite, please
+ [let us know](https://brainglobe.info/contact.html), and
+ we'd be happy to promote your paper/talk etc.**