add a README.md to massive_star (#2694)

Exec/science/massive_star/README.md

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+# massive star
+
+This setup models the convective burning shells in a massive star
+leading up to core collapse.  Some details:
+
+* The initial model is from MESA and is put onto a uniform grid using
+  the routines in the AMReX Astrophysics initial model repo
+  (https://github.com/amrex-astro/initial_models) in the
+  ``massive_star`` directory.
+
+* We use simplified-SDC together with aprox19 and an NSE table
+  (generated from pynucastro)
+
+* The Castro ``drive_initial_convection`` functionality is used to
+  establish the initial convective velocity field.