Update README.md and add submodule update (#3029)

Summary:
Without the submodule update, install_requitements would not work. Add this step in the documentation's README.md

Pull Request resolved: #3029

Reviewed By: lucylq

Differential Revision: D56087389

Pulled By: iseeyuan

fbshipit-source-id: fd96530b44f81b6dfcea07faccef06f6348fa373

Author: iseeyuan

docs/README.md

@@ -57,7 +57,11 @@ To build the documentation locally:
    ```bash
    pip3 install -r ./.ci/docker/requirements-ci.txt
    ```
+1. Update submodules
 
+   ```bash
+   git submodule sync && git submodule update --init
+   ```
 1. Run:
 
    ```bash