Documentation suggestion regarding dotfile ignoring of cosmiconfig files

[titanism]

Per #667, we should probably add to the README.md documentation something like the following:

## Linting dotfiles (e.g. cosmiconfig)

If you would like to lint files such as `.lintstagedrc.js` or `.xo-config.js`, then you will need to add the following line to your `.eslintignore` file:

echo '!.*.js' >> .eslintignore

Pinging @sindresorhus and @spence-s since you might know where to put this best?