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Document the requirement to vendor headers from Ruby #630

	name: Header check
	on: [push, pull_request]

	jobs:
	header_check:
	runs-on: ubuntu-latest
	name: Check vendored Ruby headers
	steps:
	- uses: actions/checkout@v4
	- uses: ruby/setup-ruby@v1
	with:
	ruby-version: "3.3"
	bundler-cache: true
	- run: bundle exec rake vendor_ruby_headers
	- run: git diff --no-ext-diff --ignore-submodules --exit-code