Update README.md

Author: stevenjgibbons

absolute_initial_tauy_example/README.md

@@ -1 +1,20 @@
-A directory containing a sample setrun.py file to implement the specification of a spatially varying initial yield stress using absolute values (between tauy_i and tauy_r)
+A directory containing a sample setrun.py file to implement the specification of a spatially varying initial yield stress using absolute values (between tauy_i and tauy_r)  
+
+Please look at the file **setrun.py** in this directory.
+It is *almost* identical to the **setrun.py** in the folder above but there are two crucial differences.  
+
+Firstly, we need to set the following lines near the top:  
+```
+i_use_var_tau_y = 1                    # Initial yield strength file with absolute tau values.
+fname_tau_y     = 'absolute_tau_init.tt3'  # File for the variable yield stress
+```
+
+(this is lines 28 and 29 in the **setrun.py** file in this folder).  
+
+but then, you also need to add the following two lines further down:  
+```
+    probdata.add_param('i_use_var_tau_y',i_use_var_tau_y ,'Use variablt tau_y (0,1,2)?')
+    probdata.add_param('fname_tau_y' ,  fname_tau_y      ,'File name for variable tau_y')
+```
+
+(this is lines 88 and 89 in the **setrun.py** file in this folder).