Revamp the contribution guide.

Build the document to read this one:
```bash
git fetch
git checkout origin/sean/revamp-contribution-guide # will say detached head, that's expected
./waf build
# open the built docs
```
I know it's a bit of a burden, but I'd appreciate it if everyone could actually *try* the steps in the chapter in addition to proofreading them. Thanks.
(which you will be editing)