README: document testbench Signed-off-by: Sean Cross <sean@xobs.io>

diff --git a/README.md b/README.md
index e2b0857..ffb463a 100644
--- a/README.md
+++ b/README.md

@@ -34,3 +34,13 @@
 ```
 
 When it's done, the resulting files will be in `openlane/wb_pio/runs/$CURRENT_DATE_TIME/results/final/`.
+
+## Running the testbench
+
+You can run the testbench using iverilog, which will generate a `.vcd` file:
+
+```sh
+make verify-wb_pio_test-rtl
+```
+
+You can then inspect the `.vcd` file by using a tool such as `gtkwave` to view `verilog/dv/wb_pio_test/RTL-wb_pio_test.vcd`