Add release instructiosn to CONTRIBUTING.md (#410)

CONTRIBUTING.md

@@ -16,6 +16,26 @@ To get started with contributing, please follow these steps:
 
 The team maintaining this codebase will review your PR and start a CI build for it. For security reasons, we don't automatically run CI against forked repos, and a human will review your PR prior to its CI running.
 
+## Testing
+
+There is a continuous integration pipeline on Buildkite:
+
+https://buildkite.com/buildkite/buildkite-cli
+
+## Releasing
+
+Builds on `main` include a block step to "Create a release". The step takes a tag name, then takes care of tagging the built commit.
+
+New tags trigger the release pipeline:
+
+https://buildkite.com/buildkite/buildkite-cli-release
+
+This will prepare a new draft release on GitHub:
+
+https://github.com/buildkite/cli/releases
+
+To release, edit the draft and _Publish release_.
+
 ## Reporting Issues
 
 If you encounter any issues or have suggestions for improvements, please open an issue on the GitHub repository. Provide as much detail as possible, including steps to reproduce the issue.
@@ -24,4 +44,4 @@ If you encounter any issues or have suggestions for improvements, please open an
 
 If we're really dragging our feet on reviewing a PR, please feel free to ping us through GitHub or Slack, or get in touch with [[email protected]](mailto:[email protected]), and they can bug us to get things done :) 
 
-Happy contributing!
+Happy contributing!