[Doc] Add documentation about the testbenches

TESTBENCH/README.md

@@ -2,4 +2,5 @@
 This directory contains the testbenches for FPGA fabrics that are automatically generated by OpenFPGA or tuned for a specific FPGA fabric.
 Please keep this directory clean and organize as follows:
 - Each testbench should be placed in a separated directory
+- **common**: include commonly used testbench template for post-PnR verification mainly
 - READMD is the only file allowed in the directory, others should be sub-directories.

TESTBENCH/common/README.md

@@ -0,0 +1,11 @@
+# Skywater PDK
+This directory contains the commonly used testbench template for FPGA verificatio
+
+* **post\_pnr\_fpga\_cells.v**: The netlist that includes all the standard cells used by the post-PnRed FPGA fabric
+
+* Pre-PnR testbenches
+  - **pre\_pnr\_ccff\_test.v**: The template testbench for post-PnR verification on the configuration chain 
+
+* Post-PnR testbenches
+  - **post\_pnr\_ccff\_test.v**: The template testbench for post-PnR verification on the configuration chain 
+  - **post\_pnr\_scff\_test.v**: The template testbench for post-PnR verification on the scan chain