Update CONTRIBUTING.md to include submodule cloning

[guineveresaenger]

Being unfamiliar with this repo, I thought I'd leave this hint for posterity.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.88%. Comparing base (0f99c43) to head (ec70157).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4374      +/-   ##
==========================================
- Coverage   60.91%   60.88%   -0.04%     
==========================================
  Files          91       91              
  Lines       11448    11448              
==========================================
- Hits         6974     6970       -4     
- Misses       3811     3814       +3     
- Partials      663      664       +1     

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[Graham-Pedersen]

I love this change

[pulumi-bot]

This PR has been shipped in release v3.11.0.