This plugin is not currently OSS as of February 2017 - but we are currently moving all the pieces into place for an OSS release.
The Webhook Relay Plugin publishes hooks (from an external webhook based system (e.g. Github / Dockerhub)) to subscribers on an internal network. This is primarily used for secure systems which do not expose their Jenkins instance to the outside world.
No information for the plugin 'webhook-relay' is available. It may have been removed from distribution.
Documentation can be found in the Github repo at repo
See issue tracker
Not yet released