Mention dbg/2 can be plugged into a pipeline

[Gladear]

I just noticed that plug :dbg is a thing, which I think is incredibly useful during development to help debug a pipeline.

I thought it was worth mentioning in the doc, although I see this is a "side-effect" of the way dbg/2 works, and not the actual wanted behaviour, so I can see why maintainers of both Elixir and Plug may not want to see it documented. Feel free to close the PR if that not something you want to keep 🙂

[josevalim]

I think this is great but I don't think the README is the best place to put it. Perhaps as part of the Plug.Debugger module?

[Gladear]

I agree putting this in the README feels a bit out of place. However, the Plug.Debugger module already seem to have its own specific purpose (showing an error page), and I can't seem to see a place that would seem right in the @moduledoc. Maybe in Plug.Builder?