Create README.md

Author: stevenjgibbons

relative_initial_tauy_example/README.md

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+A directory containing a sample setrun.py file to implement the specification of a spatially varying initial yield stress using values (val) between 0 and 1 in order to specify an initial tauy between tauy_i (val)=1 and tauy_r (val=0).  
+This is to say that at the start of the simulation, the value tauy is given by
+```
+tauy = tauy_r + val*(tauy_i - tauy_r)
+```