fix reference in README for exact Riemann (#2537)

Util/exact_riemann/README → Util/exact_riemann/README.md

@@ -5,13 +5,10 @@ generate exact solutions to the Riemann problem for comparison with
 Castro shocktube output.  Several inputs files for Helmholtz EOS-based
 shocktubes are provided.
 
-This solver is used in Zingale & Katz (2014), and more details are
-given there.
+This solver is used in Zingale & Katz (2015):
 
-To build the solver, simply type 'make' in this directory.
+https://ui.adsabs.harvard.edu/abs/2015ApJS..216...31Z/abstract
 
+and more details are given there.
 
-Note: at the moment, we compile in a network from Microphysics, but
-we don't use an integrator.  This is not needed since the GPackage.mak
-in the network has the network dependent code blocked out with an
-`ifneq ($(USE_REACT), FALSE)`
+To build the solver, simply type 'make' in this directory.