feat: plugin config write guide

[JimMoen]

related: emqx/emqx-plugin-template#126

[zmstone]

please also add:

1. Where should the initial config (before EMQX boot) be provided (maybe in the priv dir)
2. Where are the updated config file stored, (explain overwrite, and the bak files too?)

jim says the code is ready, just not the updated documentation. We'll merge it for the release and jim will refine it in a new pr later.