Small commit - modified CITATION.txt to include our reference, deleted...

Small commit - modified CITATION.txt to include our reference, deleted Quickstart.pdf and changes.md from top level directory.

CITATION.txt

+===========================================================================
 If you use PhysiCell in your project, please cite PhysiCell and the version 
 number, such as below:                                                      
+===========================================================================
                                                                              
 We implemented and solved the model using PhysiCell (Version 1.6.1) [1].    
                                                                              
@@ -21,4 +23,15 @@ with BioFVM [2] to solve the transport equations.
                                                                              
 [2] A Ghaffarizadeh, SH Friedman, and P Macklin, BioFVM: an efficient para- 
     llelized diffusive transport solver for 3-D biological simulations,     
-    Bioinformatics 32(8): 1256-8, 2016. DOI: 10.1093/bioinformatics/btv730  
\ No newline at end of file
+    Bioinformatics 32(8): 1256-8, 2016. DOI: 10.1093/bioinformatics/btv730 
+
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++    
++ If you use PhysiCell-X in your project, we would really appreciate if you can cite:+
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++    
+
+[1] The PhysiCell-X repository URL : https://gitlab.bsc.es/gsaxena/physicell_x
+
+[2] Saxena, Gaurav, Miguel Ponce-de-Leon, Arnau Montagud, David Vicente Dorca, and Alfonso Valencia. 
+		"BioFVM-X: An MPI+ OpenMP 3-D Simulator for Biological Systems."
+		In International Conference on Computational Methods in Systems Biology, pp. 266-279. Springer, Cham, 2021.
+ 
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