[doc] TOOLS-594 eng update README.md.

- `rm server.js`: stale worse than nothing.
- Sub-headings markdown.
- Added sections based on what's used in real Joyent repos.
At least a prose statement to make it clear the context.
Note that eng.git is valid for more than SmartOS, so I'd not add it here. Though perhaps you were shooting for demonstrating a common usage for eng.git-following repos.