Document custom registries

[phated]

No description provided.

[zladuric]

Where would I read a bit more on this? (Coming from a gulp.js issue saying helping over here ships gulp 4.0. faster :))

[phated]

The tests for https://github.com/phated/undertaker/blob/master/test/registry.js pretty much explain how they work. I've created 3 registries: https://github.com/phated/undertaker-registry (the default registry), https://github.com/phated/undertaker-common-tasks and https://github.com/undertakerjs/undertaker-task-metadata