Create project justification docs

[bobcatfish]

Expected behavior

When users come to the repo for this project, they should be able to easily find an explanation of why the project exists and how it relates to other projects.

Actual behavior

This is something we don't yet have for Tekton Pipelines, which results in questions such as tektoncd/pipeline#644. Having docs like this from the start would probably save a lot of time and answer a lot of user questions (not to mention clarify our own thoughts!).

[vtereso]

I merged an initial README to address this. Once things are fleshed out or otherwise, maybe we can docs on similar projects. This should be good to close out.

[bobcatfish]

woot let's close it 🎉