Related to #147

As a former Build user, it should be straightforward for me to migrate BuildTemplates and Builds I had previously defined to match the new model for Pipelines.

In the new model, a BuildTemplate is roughly equivalent to a Task (stateless, parameterized resources which are invoked), and a Build is roughly equivalent to a TaskRun (the running invocation, with state).

There are some differences, particularly around how Task parameters are defined and passed into steps, and how currently you can only create a Task from a pre-defined TaskRun (#249).

At a minimum we should document how to migrate to the new world, and ideally we would be able to write a script that transforms a Build YAML into a TaskRun YAML, likewise for BuildTemplate -> Task.