Update CONTRIBUTING.md for code contributors

[sandersn]

1. Add basic step-by-step startup instructions.
2. Remove/simplify some outdated instructions.
3. Rewrite some instructions to sound less like manuals from the 60s.

Prompted by Open Source Day, but a revamp has been long overdue.

[sandersn]

Some notes from talking to my wife:

1. You might not know what fork/clone are.
2. You probably don't know that Node is the standalone Javascript runtime. (It's basically the most confusing possible name.)
3. You might not know that you need to type in the monospace text to a terminal.
4. The What You'll Need step (2) contains instructions.