Create docs for each type

[bobcatfish]

Expected behavior

Users of Triggers should have a place to look for docs on each CRD that they'll be interacting with.

Actual behavior

We have no docs, but in #10 we'll be adding the types. The only docs we have at the moment are in the design doc.

Additional info

One decision we may want to make at this point is whether we want to have a markdown file per type, the way that Tekton Pipelines does, or if we want this kind of documentation to live with the code and primarily be visible at https://godoc.org/.

If we decide to rely on godocs, we need to have more carefully curated godocs than https://godoc.org/github.com/tektoncd/pipeline/pkg/apis/pipeline/v1alpha1 does, specifically:

• We need to avoid including docs here that shouldn't be user facing
• We should have inline examples
• We'd need to include docs like this https://github.com/tektoncd/pipeline/blob/master/docs/pipelines.md#ordering which aren't necessarily about individual types

It may also be harder to version godocs and present them nicely in an interface such as https://knative.dev/docs/ or https://skaffold.dev/docs/ (which i think both use netlify?)

[vtereso]

/assign

[vtereso]

I will approach this in the Tekton way with a markdown for each.