Update README.md

Author: MauroToscano

examples/l2/README.md

@@ -49,6 +49,9 @@ In Step 2, once the proof is aggregated (every 24 hours), it is verified on-chai
 2. [Foundry](https://book.getfoundry.sh/getting-started/installation)
 3. [Docker](https://docs.docker.com/engine/): for SP1 prover
 
+Submodules of the repo should be imported by running `make submodules` on the root folder
+
+
 ## Setup Holeksy
 
 #### 1. Create keystore