update README for spherical nsinker benchmark

benchmarks/nsinker_spherical_shell/README.md

@@ -5,17 +5,24 @@ The Spherical Shell NSinker benchmark
 contains the "Spherical Shell NSinker" benchmark, which is
 loosely based on the NSinker benchmark defined in
 
-Weighted BFBT Preconditioner for Stokes Flow Problems with Highly
-Heterogeneous Viscosity
-Johann Rudi, Georg Stadler, Omar Ghattas
-https://doi.org/10.1137/16M108450X
+> Weighted BFBT Preconditioner for Stokes Flow Problems with Highly Heterogeneous Viscosity,
+> Johann Rudi, Georg Stadler, Omar Ghattas,
+> SIAM Journal of Scientific Computing, Vol 39, Issue 5, 2017.
+> https://doi.org/10.1137/16M108450X
 
 but adapted to a spherical shell geometry. See the "nsinker" benchmark
 in this repository for the version in a unit box. The motivation for
 creating this benchmark was the desire to test linear solver
 performance with a spherical geometry, which is often used in practice
 and that turns out to be more challenging.
 
+This benchmark is defined and used in
+
+> Efficient distributed matrix-free multigrid methods on locally refined meshes for FEM computations,
+> Peter Munch, Timo Heister, Laura Prieto Saavedra, Martin Kronbichler,
+> ACM Transactions on Parallel Computing, 2023, 10, 1-38.
+> https://doi.org/10.1145/3580314
+
 We consider a spherical shell with inner radius 0.54 and outer radius
 1.0 with N spherical, high-viscosity, high-density sinking spheres at
 random (but predetermined in the code) locations in the