Doc: Improve contributing documentation (#499)

In the doc it mentions to go to subfolder of examples but it was not
there :).

Signed-off-by: Federico Guerinoni <guerinoni.federico@gmail.com>

Signed-off-by: Federico Guerinoni <guerinoni.federico@gmail.com>

CONTRIBUTING.md

@@ -13,6 +13,11 @@ are always appreciated!
 You can use Docker and docker-compose to test shuttle locally during development. See the [Docker install](https://docs.docker.com/get-docker/)
 and [docker-compose install](https://docs.docker.com/compose/install/) instructions if you do not have them installed already.
 
+```bash
+git clone git@github.com:shuttle-hq/shuttle.git
+cd shuttle
+```
+
 You should now be ready to setup a local environment to test code changes to core `shuttle` packages as follows:
 
 Build the required images with:
@@ -61,6 +66,8 @@ cargo run --bin cargo-shuttle -- login --api-key "test-key"
 cd into one of the examples:
 
 ```bash
+git submodule init
+git submodule update
 cd examples/rocket/hello-world/
 ```