doc: how to reply to a message

[WilliamBergamin]

Summary

This resolved #187

This PR adds documentation describing how to reply to an existing message and a unit test to ensure the behavior is maintained

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[filmaj]

Left a few suggestions for the README but other than that LGTM!

[filmaj]

Suggested change

	If you want to post a message as a reply, you can populate the `payload` with a `"thread_ts"` field. The steps after the first message posting can have `"thread_ts": "${{ steps.slack.outputs.ts }}"` in their payload. In order to reply to an existing message.
	If you want to post a message as a threaded reply, you can populate the `payload` with a `thread_ts` field. This field should equal the `ts` value of the parent message of the thread. If you want to reply to a message previously posted by this Action, you can use the `ts` output provided as the `thread_ts` of a consequent threaded reply, e.g. `"thread_ts": "${{ steps.slack.outputs.ts }}"`

[WilliamBergamin]

Committed this along with the other changes 🙇

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[zimeg]

🙌 This is great! Found something small in the payload that might need changing before a merge, but the instructions read well and I find this super helpful - I was excited to find a threaded message! 🚀

[zimeg]

🤔 I'm also open to updating the example, but no blocker at all. The deployment "started/completed" makes sense for updating a message (it makes sense for threading too), but wondering if this could post "a new commit was made" with commit details in thread instead? Or extending the current example to also reply_broadcast the message to channel?

Just some thoughts, feel free to ignore!

[fleytman]

Can u publicate new release of slack-github-action with these changes?

[WilliamBergamin]

@fleytman this PR added documentation and a unit test to the project, it did not change its behavior, you should be able to use this functionality with the existing released version

@zimeg let me know if we should cut a release, I'm not familiar with the documentation distribution

[zimeg]

@fleytman @WilliamBergamin called it with this being available in the current version v1.26.0 (and a few releases before if I had to guess). This example in the README should be good to paste into a workflow you have, taking care to note the thread_ts value!

No other fixes or enhancements have landed since this current release and this appears in the current README, so I think we're alright to wait for another release for now 🚀