Singleton Derby components are not yet documented.

[hdavidzhu]

I recently learned that Derby components can be singletons, and this doesn't seem like it's been documented yet. I'm adding this issue as a TODO so we could include it in the docs someday:

https://github.com/derbyjs/derby/blob/2c1a296e44799571e1d7386ad022ed7579aaab0a/lib/components.js#L202

Seems like the usage is just:

class MyComponent

  singleton: true

and that create is not called as part of the lifecycle. More context would be greatly appreciated 😄