At the time of writing this response the features timeline reflects yaml releases are coming 2018 Q3.

https://learn.microsoft.com/en-us/azure/devops/release-notes/

Update: This has been bumped a few times. Checking the comments below is recommended as folks have been providing updates as they find them.

Update

As per comments, this is now possible:
https://devblogs.microsoft.com/devops/whats-new-with-azure-pipelines/. The following is copied and pasted from the article and demonstrates using various stages:

stages:
- stage: Build
  jobs:
  - job: Build
    pool:
      vmImage: 'Ubuntu-16.04'
    continueOnError: true
    steps:
    - script: echo my first build job
- stage: Deploy
  jobs:
    # track deployments on the environment
  - deployment: DeployWeb
    pool:
      vmImage: 'Ubuntu-16.04'
    # creates an environment if it doesn’t exist
    environment: 'smarthotel-dev'
    strategy:
      # default deployment strategy
      runOnce:
        deploy:
          steps:
          - script: echo my first deployment