Add more docs on Triggers usage

[qu1queee]

Idea:

Per todays shipwright-io/community#119 we concluded that this repository requires more documentation focus on different features or use-cases.

For example, how does a Tekton Pipeline looks when SHP is enable there.

[qu1queee]

Per the refinement meeting today, this should be a pre-req for shipping the first Triggers release.

[qu1queee]

@jkhelil per refinement, fyi!

[adambkaplan]

From refinement session - we are going to try and release Triggers with Builds v0.15.0 as its baseline. We are going to use the v0.15.0 version number to align with the respective Builds version.

As a first release, we will absolutely need documentation in this repository.

[adambkaplan]

After reviewing the current state of the code, we have two pieces of SHIP-0031 implemented:

1. Trigger build within a pipeline, using Tekton's Custom Tasks feature.
2. Trigger a build after a Tekton pipeline completes.