docs: document createFetch custom fetch option

[poetryofcode]

Summary

Fixes #533

The createFetch function accepts a fetch option in CreateFetchOptions that lets you provide a custom fetch implementation. This is useful for Node.js-compatible fetch (e.g. undici), service worker fetch, or test mocks — but it was completely undocumented in the README.

Changes

• README.md: Added a new "🔧 Using a custom fetch implementation" section after "Create fetch with default options", showing how to use createFetch({ fetch: customImpl }) with a basic example and a practical undici example.

Test plan

• README renders correctly on GitHub and npmjs.com

Summary by CodeRabbit

• Documentation
  • Added guidance for using a custom fetch implementation when creating an ofetch instance.
  • Included examples for passing a custom fetch function and using undici’s fetch in Node.js.
  • Expanded the setup docs with this new option alongside the default configuration example.

[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: 25d4d2d3-9d26-4410-90c3-79ed29745c8a

📥 Commits

Reviewing files that changed from the base of the PR and between 9102908 and 8fa6e11.

📒 Files selected for processing (1)

• README.md

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📝 Walkthrough

Walkthrough

A new "Using a custom fetch implementation" subsection is added to README.md, documenting createFetch({ fetch }) with examples for a generic custom fetch function and undici's fetch in Node.js.

Changes

Custom fetch documentation

Layer / File(s)	Summary
createFetch custom fetch section README.md	Adds a subsection with two code examples showing how to wire ofetch to a caller-provided fetch implementation, including undici usage for Node.js.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

A rabbit hops through Node.js land,
With undici fetch close at hand.
"No docs?" it cried, then wrote them clear,
createFetch({ fetch }) — now have no fear!
The README blooms, the gap is filled. 🐇

🚥 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 doc update for documenting the custom fetch option.
Linked Issues check	✅ Passed	The README now documents createFetch({ fetch }) as requested in issue #533, including example usages.
Out of Scope Changes check	✅ Passed	The changes are limited to README documentation and stay within the issue's 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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