Test Case for Stochastic Backscatter Model (#187)

* Create README

* Add backward-facing step test case

* Delete backscatter/backward_step/README

* Update README

Added a note about initial DDES simulation for backscatter model.

Author: AngPass

backscatter/backward_step/README

@@ -0,0 +1 @@
+It is advisable to perform an initial DDES simulation with the backscatter model turned off in order to allow the shielding function to form correctly.