Better docs on writing custom audits

**Feature request summary**
I'd like better docs explaining how to write custom audits. Yes I've seen the docs, but 1) your custom audit example gives an error in the gatherer, 2) I'd like high level instructions on how to write arbitrary audits. API docs for this?

I am willing to contribute these docs if I could figure out how to write my own gatherers and audits. I've read the source code and I still find it very confusing. Axe audits vs regular audits? Am I supposed to use the chrome debugging API? What about just simple querying of the DOM?

My organization would like to adopt Ligthhouse, but we need to write our own tests.

Others could use clearer docs without having to delve into the source code.