Update the citation guide

README.rst

@@ -35,6 +35,12 @@ If you use :code:`bilby` in a scientific publication, please cite
 * `Bilby: A user-friendly Bayesian inference library for gravitational-wave
   astronomy
   <https://ui.adsabs.harvard.edu/#abs/2018arXiv181102042A/abstract>`__
+* `Bayesian inference for compact binary coalescences with BILBY: validation and application to the first LIGO-Virgo gravitational-wave transient catalogue <https://ui.adsabs.harvard.edu/abs/2020MNRAS.499.3295R/abstract>`__
+
+The first of these papers introduces the software, while the second introduces advances in the sampling approaches and validation of the software.
+If you use the :code:`bilby_mcmc` sampler, please additionally cite
+
+* `BILBY-MCMC: an MCMC sampler for gravitational-wave inference <https://ui.adsabs.harvard.edu/abs/2021MNRAS.507.2037A/abstract>`__
 
 Additionally, :code:`bilby` builds on a number of open-source packages. If you
 make use of this functionality in your publications, we recommend you cite them