Update plugins.md
Update plugins.md

Added a simple deploy-webhook plugin that is configured purely through a config:set
progrium/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,progrium/dokku,progrium/dokku,progrium/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,progrium/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku