contributing: note how to install foundry

[tynes]

Description
Add directions on how to install foundry to the contributing doc

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Latest commit: c951b7a

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[smartcontracts]

Pls also add to dependencies: https://github.com/ethereum-optimism/optimism/blob/develop/CONTRIBUTING.md#dependencies

[tynes]

Pls also add to dependencies: https://github.com/ethereum-optimism/optimism/blob/develop/CONTRIBUTING.md#dependencies

addressed, good catch

[smartcontracts]

LGTM ty

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