Create a working validate example

[ncskier]

Expected Behavior

Now that #89 has been merged, Triggers has the capability to validate http POST events. For someone learning about Triggers, I expect there to be example YAML files that I can apply to create a working example of this validation.

Actual Behavior

There is nothing in the examples/ directory that creates an EventListener with working validation.

Additional Info

It could be very helpful to have an accompanying README, like the one created in #113.

[khrm]

/assign khrm

[ncskier]

Thanks for picking this up @khrm! 🙂

[ncskier]

@vtereso pointed out that the validate-event.md page has a static example, so under the issue, please update the page to use your new validation example with <!-- FILE: examples/path/to/new/validate-example.yaml --> and run ./hack/update-docs.sh

[akihikokuroda]

The validate-github-event.yaml has issue #119