Docs: add "yarn postinstall" to the readme

[katspaugh]

What it solves

Resolves #3164

How this PR fixes it

Add instructions about running yarn postinstall.

[github-actions]

Branch preview

✅ Deploy successful!

https://update_readme--walletweb.review-wallet-web.5afe.dev/home?safe=eth:0xA77DE01e157f9f57C7c4A326eeE9C4874D0598b6

[github-actions]

ESLint Summary View Full Report

Annotations are provided inline on the Files Changed tab. You can also see all annotations that were generated on the annotations page.

Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[github-actions]

📦 Next.js Bundle Analysis for safe-wallet-web

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	78.82% (-0.02% 🔻)	11285/14317
🔴	Branches	56.64% (-0.02% 🔻)	2507/4426
🟡	Functions	63.08% (-0.04% 🔻)	1796/2847
🟢	Lines	80.18% (-0.02% 🔻)	10178/12694

Show files with reduced coverage 🔻

St.❔	File	Statements	Branches	Functions	Lines
🟢	... / index.tsx	82.35% (-8.82% 🔻)	66.67% (-16.67% 🔻)	50% (-12.5% 🔻)	83.87% (-9.68% 🔻)

Test suite run success

1384 tests passing in 186 suites.

Report generated by 🧪jest coverage report action from 3a52d03

[katspaugh]

Solved in #3167