Zapier: Fix broken image link and expand on the examples and overview

[robertjahn]

Currently the listing has minimal details on what Zapier is and the reason a user would use this integration. This listing should show that Keptn is more than just a trigger to an external platform and briefly explain why Keptn does versus Zapier does for orchestrating.

Some suggestions:

• Intro on what Zapier, why use it, why Keptn + Zapier is better togethe
• URL their site
• Prerequisites section with details
• Feedback section
• Troubleshooting section
• Clearly broken out use cases. Now you have have both a trigger of a keptn sequence and a webhook.
• For webhooks need to explain the subscription setup and details on making it work as a passive or active model.

Ease use cases example should have:

• How to configure it on the Zapier side and Keptn side.
• Screenshots (of config as relevant)
• Code snippet (for example there is no example of the webhook configuration)
• Perhaps a screenshots too for the output

[christian-kreuzberger-dtx]

No issue description, issue is stale for 17 days - closing it.

[TannerGilbert]

Actually this should be fixed by #74

[robertjahn]

Hi re-opening for this is not related to #74 . I will add a better description.