Introductory documentation

[weberc2]

• What is this project?
• What are its goals and motivation?
• What is its stage of development?
• Basic usage documentation

[weberc2]

Some objective ideas: builder should be maintainable, friendly to contributors, and easy to extend. Problems I've encountered with pants and bazel are that much of the code in those code bases is hard to understand and extending is difficult. It takes a lot of familiarity with the broader code base (it often doesn't suffice to understand the plugin interface) in order to write useful, correct plugins and the documentation (for writing non-toy plugins, anyway) is lacking. A contributor (who is familiar to Go, although Go itself is famously easy to learn in a short time) should be able to go from zero to writing a useful plugin in a weekend or sooner (this is perhaps a poor metric, since much of the difficulty in writing plugins is understanding the nuances plugin's domain; however, at least it conveys the aspiration).