Add documentation around ignoring js files

https://trello.com/c/N8xHlYlK/
I see the fact that we expose and advertise `JSHint` as well as `RuboCop` as a disadvantage. It was necessary because we needed to provide configuration to our customers fast, thus this decision was made. However, we should move in the direction of treating the linters as implementation details, free to be broken at any time, not public API.
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.

Can you explain a bit more why you think it's worth advertising the `.jshintignore` file?

If we decide to keep and document default `.jshintignore` file, why even have the `ignore_file` option in `.hound.yml` if we can just tell people to create `.jshintignore`?