doc: add steps to add new plugin in CONTRIBUTING.md

[horriblename]

Took me a while to figure this out

Should we rewrite/regenerate the table of contents in CONTRIBUTING.md to include the "keybinds" and "adding plugins" subsections as well?

[NotAShelf]

I am not exactly sure if we should include keybind and plugin addition instructions in CONTRIBUTING.md - I was thinking of reserving a page for development guides on the documentation site and keep CONTRIBUTING.md strictly for guidelines for style, commits and so on.

The downside of the static documentation is that it's asciidoc over markdown, which is slightly harder to contribute to; but offers more possibilities for organization and styling.

[horriblename]

I'd rather use markdown as well but I think putting it on the doc site makes more sense - and probably easier to find as a reader

[NotAShelf]

I'll move the keybinds guide to the documentation. We can refactor this PR to move the new plugin guide to it as well.

[NotAShelf]

I'll move the keybinds guide to the documentation. We can refactor this PR to move the new plugin guide to it as well.

this will take a little longer than expected as I'm also refactoring the documentation in the process, should be done in a day or two

[horriblename]

@NotAShelf refactor done

[NotAShelf]

The section indicator is missing, but no trouble. I'll add it.

LGTM