Overwriting existing files: example in the documentation.

[mjordan79]

I think that using the append directive is not immediate, so a small example in the documentation should be provided

Example documentation: https://coreos.github.io/butane/examples/

Snippet to explain: variant: fcos version: 1.5.0 storage: files:

• path: /etc/sudoers overwrite: false append:
  • inline: | core ALL=(ALL) NOPASSWD: /usr/bin/docker