Adding a citation guide for bilby

Following discussions in the past couple bilby development calls, this MR adds a citation guide for bilby.

Note: citations are perhaps still incomplete, and I'm happy to take suggestions for what to add in or take out.

Also, should we add in a similar citation guide in bilby_pipe?

docs/citing-bilby.txt

+=======================================
+Acknowledging/Citing Bilby
+=======================================
+
+If you have used Bilby in your scientific work, please acknowledge us in your papers/proposals. 
+
+ .. code:: bibtex
+
+    @article{bilby_paper,
+        author = "Ashton, Gregory and others",
+        title = "{BILBY: A user-friendly Bayesian inference library for gravitational-wave astronomy}",
+        eprint = "1811.02042",
+        archivePrefix = "arXiv",
+        primaryClass = "astro-ph.IM",
+        doi = "10.3847/1538-4365/ab06fc",
+        journal = "Astrophys. J. Suppl.",
+        volume = "241",
+        number = "2",
+        pages = "27",
+        year = "2019"
+    }
+
+Additionally, if you used `bilby_pipe` or `parallel_bilby`, please cite appropriate papers mentioned below"
+
+ .. code:: bibtex
+
+    @article{bilby_pipe_paper,
+        author = "Romero-Shaw, I. M. and others",
+        title = "{Bayesian inference for compact binary coalescences with bilby: validation and application to the first LIGO\textendash{}Virgo gravitational-wave transient catalogue}",
+        eprint = "2006.00714",
+        archivePrefix = "arXiv",
+        primaryClass = "astro-ph.IM",
+        doi = "10.1093/mnras/staa2850",
+        journal = "Mon. Not. Roy. Astron. Soc.",
+        volume = "499",
+        number = "3",
+        pages = "3295--3319",
+        year = "2020"
+    }
+
+    @article{pbilby_paper,
+        author = "Smith, Rory J. E. and Ashton, Gregory and Vajpeyi, Avi and Talbot, Colm",
+        title = "{Massively parallel Bayesian inference for transient gravitational-wave astronomy}",
+        eprint = "1909.11873",
+        archivePrefix = "arXiv",
+        primaryClass = "gr-qc",
+        reportNumber = "LIGO Document P1900255-v1",
+        doi = "10.1093/mnras/staa2483",
+        journal = "Mon. Not. Roy. Astron. Soc.",
+        volume = "498",
+        number = "3",
+        pages = "4492--4502",
+        year = "2020"
+    }
+
+If you use any of the accelerated likelihoods like `ROQGravitationalWaveTransient`, `MBGravitationalWaveTransient`, `RelativeBinningGravitationalWaveTransient` etc., please cite the following papers in addition to the above.
+
+- `ROQGravitationalWaveTransient`
+ .. code:: bibtex
+
+     @article{roq_paper,
+        author = {Smith, Rory and Field, Scott E. and Blackburn, Kent and Haster, Carl-Johan and P\"urrer, Michael and Raymond, Vivien and Schmidt, Patricia},
+        title = "{Fast and accurate inference on gravitational waves from precessing compact binaries}",
+        eprint = "1604.08253",
+        archivePrefix = "arXiv",
+        primaryClass = "gr-qc",
+        reportNumber = "LIGO-DOCUMENT-NUMBER-P1600096, LIGO-P1600096",
+        doi = "10.1103/PhysRevD.94.044031",
+        journal = "Phys. Rev. D",
+        volume = "94",
+        number = "4",
+        pages = "044031",
+        year = "2016"
+    }
+
+- `MBGravitationalWaveTransient`
+ .. code:: bibtex
+
+    @article{mb_paper,
+        author = "Morisaki, Soichiro",
+        title = "{Accelerating parameter estimation of gravitational waves from compact binary coalescence using adaptive frequency resolutions}",
+        eprint = "2104.07813",
+        archivePrefix = "arXiv",
+        primaryClass = "gr-qc",
+        doi = "10.1103/PhysRevD.104.044062",
+        journal = "Phys. Rev. D",
+        volume = "104",
+        number = "4",
+        pages = "044062",
+        year = "2021"
+    }
+
+
+- `RelativeBinningGravitationalWaveTransient`
+ .. code:: bibtex
+
+    @article{relbin_paper,
+        author = "Cornish, Neil J.",
+        title = "{Fast Fisher Matrices and Lazy Likelihoods}",
+        eprint = "1007.4820",
+        archivePrefix = "arXiv",
+        primaryClass = "gr-qc",
+        month = "7",
+        year = "2010"
+    }   
+
+Please also ensure that you appropriately cite the following:
+
+- The sampler(s) used for your analysis
+- Any additional package(s) that you use to generate waveforms e.g. `lalsuite` (https://doi.org/10.7935/GT1W-FZ16)
+as well as the original scientific papers for each waveform.