OpenSCD-core plug-in guide

[Sander3003]

As a developer
I would like to know some guidelines how to build a OpenSCD-core plugin
So I can build a OpenSCD-core plug-in more easyly

Acceptance criteria:
Plug-in guide (simular to the component guide)

[Sander3003]

@pascalwilbrink do you have a url to the component guide?

[Sander3003]

https://p-wilbrink.github.io/openscd-github/

[Sander3003]

https://github.com/openscd/.github/blob/Add_Component_Development_guide/docs/component_guide.md

[Sander3003]

https://gist.github.com/pascalwilbrink/8735202dad95c324d19a79373938a7eb