Add comment reminding devs to install the submodule

[benjl]

Submodule doesn't install automatically when first cloning the repo - this comment reminds you if you need to do that

Checks

• I have thoroughly tested all of the code I have modified/added/removed to ensure something else did not break
• I have followed semantic commit messages e.g. feat: Add foo, chore: Update bar, etc...
• My branch has a clear history of changes that can be easy to follow when being reviewed commit-by-commit
• My branch is functionally complete; the only changes to be done will be those potentially requested in code review
• All changes requested in review have been fixuped into my original commits.
• Fully tokenized all my strings (no hardcoded English strings!!) and supplied bulk JSON strings below