Document how to authenticate with Docker on Travis

[roborourke]

Due to Docker's new rate limits we need to inform clients and partners how to set up Travis up to use an authenticated account to avoid the problem.

Thanks to @nathanielks - the steps are:

• Create an account on Docker Hub
• Create an Access Token in Docker Hub
• Add an environment variable named DOCKER_USERNAME to your Travis environment with the value of your Docker Hub username.
• Add an environment variable named DOCKER_PASSWORD to your Travis environment with the value of the Access Token created before.
• Add a before_install section of your .config/travis.yml file:

before_install:
  - echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin

• Commit and push these changes to a branch to trigger a build.

We can also suggest they make it a pro account to avoid the rate limits unless we decide to suggest people make a support request to us to set this up using the HM account.