Added required configuration options to the Giant impact example.

examples/GiantImpacts/GiantImpact/README.md

 An example planetary simulation of a giant impact onto the young Uranus with 
 ~10^6 SPH particles, as described in Kegerreis et al. (2018), ApJ, 861, 52.
+
+This example requires the code to be configured to use the Planetary
+hydrodynamics and equation of state:
+ --with-hydro=planetary --with-equation-of-state=planetary