docs: document custom fetch option

[vittorioexp]

Summary

Closes #533.

Documents the fetch option for createFetch and ofetch.create, which lets callers plug in a custom Fetch API implementation.

Test plan

• Verified against src/fetch.ts and CreateFetchOptions in src/types.ts
• README-only change

Summary by CodeRabbit

• Documentation
  • Added guidance for configuring a custom fetch implementation.
  • Included examples using supported configuration methods.
  • Documented the required standard fetch(input, init?) signature and compatibility with existing features such as retries, hooks, and JSON body handling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: fd8251a4-fed6-48fa-bee4-f898f9f07940

📥 Commits

Reviewing files that changed from the base of the PR and between 1dbc37f and aa9a74d.

📒 Files selected for processing (1)

• README.md

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📝 Walkthrough

Walkthrough

Changes

Custom fetch documentation

Layer / File(s)	Summary
Document custom fetch configuration README.md	Adds examples and guidance for supplying a custom fetch through createFetch or ofetch.create, including the required standard signature and preserved ofetch features.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: maxtaran2010

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the README docs update for the custom fetch option.
Linked Issues check	✅ Passed	The README now documents passing a custom fetch function, matching issue #533.
Out of Scope Changes check	✅ Passed	The change is limited to README documentation and stays within the stated scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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