Add documentation around ignoring js files

https://trello.com/c/N8xHlYlK/
> For example, if we move away from JSHint we'll need to support the default .jshintignore file going forward, to avoid breaking changes for our users. If we don't advertise that .jshintignore is picked up by default, we don't incur this penalty.

Good point. I'm sold.