docs: add usage example for SLACK_THREAD_TS to reply in Slack threads (#210)

[BhuvaneshAdithya45]

This PR updates the README.md to include documentation on using the SLACK_THREAD_TS environment variable to reply to a specific Slack thread.

Changes

• Added clear step-by-step instructions for setting SLACK_THREAD_TS.
• Included a sample workflow snippet for quick reference.

Why

Helps users easily send Slack notifications as replies within an existing thread, improving conversation organization.

Fixes #210

[BhuvaneshAdithya45]

Hi @mrrobot47 ,@Lord-Zero,
I’ve added documentation with an example for SLACK_THREAD_TS to help users reply within Slack threads more easily (Fixes #210).
All checks have passed — could you please review when you have a moment?
Thank you!

[notnmeyer]

this appears to be nonsense. there is no ts output set by the action to be reading in a later step.