Add a note for contributors installing fourmolu #1327

	name: Validate Skip

	# This Workflow is special and contains a workaround for a known limitation of GitHub CI.
	#
	# The problem: We don't want to run the "validate" jobs on PRs which contain only changes
	# to the docs, since these jobs take a long time to complete without providing any benefit.
	# We therefore use path-filtering in the workflow triggers for the validate jobs, namely
	# "paths-ignore: doc/**". But the "Validate post job" is a required job, therefore a PR cannot
	# be merged unless the "Validate post job" completes succesfully, which it doesn't do if we
	# filter it out.
	#
	# The solution: We use a second job with the same name which always returns the exit code 0.
	# The logic implemented for "required" workflows accepts if 1) at least one job with that name
	# runs through, AND 2) If multiple jobs of that name exist, then all jobs of that name have to
	# finish successfully.
	on:
	push:
	paths:
	- 'doc/**'
	- '**/README.md'
	- 'CONTRIBUTING.md'
	- "changelog.d/**"
	# only top level for these, because various test packages have them too
	- "*/ChangeLog.md"
	- "*/changelog.md"
	- "release-notes/**"
	branches:
	- master
	pull_request:
	paths:
	- 'doc/**'
	- '**/README.md'
	- 'CONTRIBUTING.md'
	- "changelog.d/**"
	- "*/ChangeLog.md"
	- "*/changelog.md"
	- "release-notes/**"
	release:
	types:
	- created

	jobs:
	validate-post-job:
	if: always()
	name: Validate post job
	runs-on: ubuntu-latest
	steps:
	- run: exit 0

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