docs: readme tweak

[bryanchriswhite]

Changes

• remove redundant yarn install instruction

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Code Review Checklist (to be filled out by reviewer)

• [ ] Description accurately reflects what changes are being made.
• [ ] Either the PR references an issue (via the "Development" combobox) or the description explains the need for the changes.
• [ ] The PR appropriately sized.
• [ ] The PR contains an idempotent DB migration.
• [ ] I have verified the correctness of the DB migration using relevant data (e.g. test-generated data).
• [ ] New code has enough tests.
• [ ] New code has enough documentation to answer "how do I use it?" and "what does it do?".
• [ ] Existing documentation is up-to-date, if impacted.