Add plugin template explanation

I think it should be a little clearer what is the purpose of the `blueprint.yaml`

Generally, when you use the local workflows API you also have to supply a blueprint to process. The `blueprint.yaml` provided in the test already maps the operation in `tasks.py` to a `cloudify.interface.lifecycle` operation. This is intentional and should help users fill the gap between the operation they define and the blueprint. `blueprint.yaml` is also heavily documented to help a beginner user discover other features of the blueprint as well.
Another thing that is worth highlighting is the actual use of the local workflows API that happens in `test_plugin.py`. You already mention it in your documentation but it seems to be skimmed through instead of highlighted (in my opinion). The test show an example of instantiating a local workflow execution environment and executing arbitrary workflows with it (`install` in the case of the test).