Update README: only clone OpenROAD submodule
Designs should be pulled in as needed, but OR is needed for build.
@@ -50,7 +50,7 @@ To install from the active developer repository.
```sh
$ git clone https://github.com/siliconcompiler/siliconcompiler
$ cd siliconcompiler
-$ git submodule update --init --recursive
+$ git submodule update --init --recursive third_party/tools/openroad
$ pip install -r requirements.txt
$ python -m pip install -e .
```
