adding generate-spreadsheet.md

docs/generate-spreadsheet.md

+Auto-generation of DLB Spreadsheets
+===================================
+
+Introduction
+------------
+
+This manual explains how to automatically generate the set of spreadsheets that
+define a DLB and are required as input to the pre-processor.
+
+This tool comes handy in the following scenarios:
+
+    * a DLB for a new turbine needs to be generated;
+    * a different wind turbine class needs to be evaluated;
+    * a new parameter needs to be included in the htc file;
+    * different parameters variations are required, e.g. different wind speed
+    range or different number of turbulent seed.
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