Add preprint to readme

README.md

@@ -25,7 +25,7 @@ We require:
 
 The library **polyDEAL** employs **deal.II** as main third-party library. As **deal.II** itself depends on other external libraries for many functionalities, we strongly suggest to download and install deal.II following the instructions available at https://www.dealii.org/download.html and https://www.dealii.org/developer/readme.html. The minimal set of other external libraries that we require are: **METIS**, **p4est**, **Trilinos**. All of them should be compiled against  MPI during the installation phase of deal.II. 
 
-While *METIS* is generally used to partition a triangulation among several processors, in the context of polytopal methods it is heavily employed as an agglomeration strategy to build polytopic elements out of fine grids composed by standard shapes. *Trilinos* (in particular its multilevel solvers and distributed matrices) is employed as main parallel linear algebra library. We also support novel agglomeration strategies based on the R-tree spatial data structure.
+While *METIS* is generally used to partition a triangulation among several processors, in the context of polytopal methods it is heavily employed as an agglomeration strategy to build polytopic elements out of fine grids composed by standard shapes. *Trilinos* (in particular its multilevel solvers and distributed matrices) is employed as main parallel linear algebra library. We also support novel agglomeration strategies based on the R-tree spatial data structure. The associated preprint can be found online on [arXiv](https://arxiv.org/pdf/2404.18505).
 
 
 To enable to computation of some quality metrics, mostly of theoretical interests and not really relevant in application codes, the external library **CGAL** is required. As this is a dependency of *deal.II* as well, it is sufficient to configure deal.II with it.