Document in Readme how to use `helmfiles` with Docker

## what

For example, do this...

```
FROM cloudposse/helmfiles:0.1.1 as helmfiles
....
COPY --from=helmfiles /helmfiles.d/ /conf/helmfiles.d/
```

## why
* We recently moved helmfiles from `cloudposse/geodesic` into a separate repo

## references
* https://github.com/cloudposse/testing.cloudposse.co/pull/14/files#diff-3254677a7917c6c01f55212f86c57fbf