[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.
Maybe there is a better way to present this, but I want to help people driving by one of our repos that SmartOS is the development and test environment.

This information may also become more relevant if we have services on lx branded zones.

How would you like this to be presented?