document how to use `@` with `--data`

It is not obvious how to specify a file with the --data attribute. The code and specs indicate it should be prefixed with an @:

pact_broker-client/lib/pact_broker/client/cli/webhook_commands.rb

Line 86 in 3a342de

if body && body.start_with?("@")

For example, this example shows how it could be used IRL:

 	pact-broker create-or-update-webhook \
	  'https://api.github.com/repos/pactflow/example-consumer/statuses/$${pactbroker.consumerVersionNumber}' \
	  --header 'Content-Type: application/json' 'Accept: application/vnd.github.v3+json' 'Authorization: token $${user.githubCommitStatusToken}' \
	  --request POST \
	  --data @${PWD}/pactflow/github-commit-status-webhook.json \
	  --uuid ${GITHUB_WEBHOOK_UUID} \
	  --consumer ${PACTICIPANT} \
	  --contract-published \
	  --provider-verification-published \
	  --description "Github commit status webhook for ${PACTICIPANT}"

or inlined (without the @ usage):

	pact-broker create-or-update-webhook \
	  "https://api.github.com/repos/${GITHUB_REPO}/dispatches" \
	  --header 'Content-Type: application/json' 'Accept: application/vnd.github.everest-preview+json' 'Authorization: Bearer $${user.githubToken}' \
	  --request POST \
	  --data '{ "event_type": "contract_requiring_verification_published","client_payload": { "pact_url": "$${pactbroker.pactUrl}", "sha": "$${pactbroker.providerVersionNumber}", "branch":"$${pactbroker.providerVersionBranch}" , "message": "Verify changed pact for $${pactbroker.consumerName} version $${pactbroker.consumerVersionNumber} branch $${pactbroker.consumerVersionBranch} by $${pactbroker.providerVersionNumber} ($${pactbroker.providerVersionDescriptions})" } }' \
	  --uuid ${CONTRACT_REQUIRING_VERIFICATION_PUBLISHED_WEBHOOK_UUID} \
	  --provider ${PACTICIPANT} \
	  --contract-requiring-verification-published \
	  --description "contract_requiring_verification_published for ${PACTICIPANT}"

We should update the description to improve this:

  -d, [--data=DATA]        # Webhook payload (file or string)

@mefellows if you remember the person who asked about this, please see if this makes more sense to them.

Oh, I found the thread.

Awesome, thanks Beth. I think that clarifies it nicely.