Pipeline as code (YAML) preview

This post previews support for defining VSTS Build pipelines using YAML files checked into source repositories. This allows CI definitions to evolve alongside code, enabling source-centric workflows like pull requests and code reviews for CI. Users opt-in to the 'Build YAML definitions' preview feature and create a .vsts-ci.yml file in their Git repository root. Committing this file automatically configures a new build definition that runs on changes to the master branch. The preview currently supports CI, with plans to bring CD online later. It utilizes the existing VSTS automation system, supporting all inbox, custom, and marketplace tasks on both private and hosted agents.