Add a help text to the Slack Service
Add a help text to the Slack Service

When setting up the service I was not sure which webhook I should use, by adding
this Help text I hope we can help people in the future with setting up this hook

Signed-off-by: Jeroen van Baarsen <2e488d5cebaf6d2832884be70e328933bd4c80ab@gmail.com>

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