Add branch strategy to README #1459

	on: pull_request

	jobs:
	validate:
	name: Validate build
	runs-on: ubuntu-latest
	steps:
	- name: Checkout spdx-spec
	uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 #v4.2.2
	with:
	fetch-depth: 1
	- name: Set up specific Python version
	uses: actions/setup-python@0b93645e9fea7318ecaed2b359559ac225c90a2b #v5.3.0
	with:
	python-version: "3.12"
	cache: "pip"
	- name: Install pre-requisites
	run: pip install -r requirements.txt
	- name: Build the site to validate (use mkdocs.yml, no model files, no PDF export)
	run: ENABLE_PDF_EXPORT=0 mkdocs build --clean --verbose
	# To validate the PR from within spdx-spec repo, build using mkdocs.yml
	# (no model files) and without the time-consuming PDF export

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