YAML build pipeline creation experience is in preview. (today is 2018-12-04)
YAML for release pipelines seems to be a ways off still: 2019 Q2
Preview features can be enabled from your profile like this:
EDIT: As nullforce points out in comments, this only enables a YAML experience for build pipelines and not release pipelines.
UPDATE (2019-05-16): Following Microsoft's "Build 2019", the full YAML experience for both build and deployment should now be possible in the same YAML pipelines file.
