Add documentation around ignoring js files

https://trello.com/c/N8xHlYlK/
Are you suggesting not mentioning that we'll automatically pick up a `.jshintignore` file if it exists? I understand not wanting to marry to specific linters, however we're already explaining that we use `JSHint` under the hood and to configure it you need to load a `JSHint` config file. Especially since we are looking to make it easier for users who use the specific linter locally.