README.md

* Multiple files are now supported across operations.
  * Can be specified as a list or a comma separated string.
  * Single item per task params will still work without changes.
* Added `files`, `filenames`, and `file`, as aliases for the `filename` param.
* Improved output of error message to always include stderr
* `exists` now supports checking files

Follow up PRs encouraged across roles to start converting `with_items` loops on `kube` tasks into `files` param lists so we can improve performance.