docs: Provide Dockerfile example for validating Cloud-Init user data

Getting cloud-init user data correct may prove difficult for users. The following Dockerfile can be used to validate the user data:

FROM ubuntu:latest

RUN apt-get update -y && \
    apt-get install -y cloud-init

ENTRYPOINT [ "cloud-init", "schema", "--config-file", "/cloud-config.yaml" ]

Let's imagine we build the following image from the above Dockerfile:

docker build -t civ .

Now, imagine the following cloud config file exists as "my-config.yaml":

#cloud-config
ssh_pwauth: true
users:
    - default
    - name: vmware
      sudo: ALL=(ALL) NOPASSWD:ALL
      lock_passwd: false
      passwd: '$1$salt$SOC44fVbA/ZxeIwD5yw1u1'
      shell: /bin/bash
write_files:
    - content: |
      "VMSVC Says Hello World"
      path: /helloworld

Let's run the validator on the above file:

$ docker run -it --rm -v $(pwd)/my-config.yaml:/cloud-config.yaml:ro civ
Error:
Cloud config schema errors: format-l13.c7: File /cloud-config.yaml is not valid yaml. while scanning a simple key
  in "<byte string>", line 13, column 7:
          "VMSVC Says Hello World"
          ^
could not find expected ':'
  in "<byte string>", line 14, column 7:
          path: /helloworld
          ^

We should add documentation to this repo's doc area with the above example.