Update README.md

README.md

@@ -16,6 +16,7 @@ cd skywater-pdk
 The cell libraries are in submodules that need to be checked out independently:
 ```
 git submodule update --init libraries/sky130_fd_sc_hd/latest
+git submodule update --init libraries/sky130_fd_pr/latest
 ```
 To create liberty files, go into the `skywater-pdk` directory and do:
 ```