General: Prefix commit messages with "plugin name: "

[SimonVrouwe]

At least for plugins (directly) inside this repo, I think we should try to stick to the format lightning uses, something like:

plugin name: One subject line
         (empty line)
more detailed description (if any)

[ca-ruz]

@chrisguida This looks like it can be closed.

[chrisguida]

Oh hmm, this is just a comment on what commits should look like... I definitely agree and have actually been requiring this on recent PRs.

Shall we document this somewhere?

[ca-ruz]

Created PR: #617 with suggested changes.