Update contributing guidelines for submodule initialization

Author: mirams

CONTRIBUTING.md

@@ -8,10 +8,11 @@ For instructions on installing as a user, see [README.md](README.md)
 Developers should:
 
 1. Clone the repository
-2. Create a virtual environment, using e.g. virtualenv or conda. Make sure to use Python3 (e.g. `$ virtualenv venv -p python3`). (If you are on windows you might need to install virtual env first with `pip install virtualenv`. Make sure your python3 installation is in your path.)
-3. Activate the environment (e.g. `$ source venv/bin/activate`). (On Windows, virtualenv creates a batch file to activate the virtualenv: `\path\to\env\Scripts\activate`)
-4. Install the developer requirements into the virtual environment: `pip install -r dev-requirements/dev.txt`
-5. Run the tests: `$ python -m pytest`.
+2. Run `git submodule init` and `git submodule update` as some subfolders that are required are from other git repos.
+3. Create a virtual environment, using e.g. virtualenv or conda. Make sure to use Python3 (e.g. `$ virtualenv venv -p python3`). (If you are on windows you might need to install virtual env first with `pip install virtualenv`. Make sure your python3 installation is in your path.)
+4. Activate the environment (e.g. `$ source venv/bin/activate`). (On Windows, virtualenv creates a batch file to activate the virtualenv: `\path\to\env\Scripts\activate`)
+5. Install the developer requirements into the virtual environment: `pip install -r dev-requirements/dev.txt`
+6. Run the tests: `$ python -m pytest`.
 
 ### Requirements