Document how to use Dependabot to keep a test suite submodule up to date

[Julian]

(Via @Relequestual here)

Implementers may appreciate a GitHub action which automatically sends PRs to their repo to update the test suite.

As a concrete example, python-jsonschema has a git-subtree of this suite here, and would benefit from automated pull requests whenever the main branch of this repository is updated.

To do this in general, such a github action would need to be configured with the path within a repo where the subtree lives, likely need to support both git-subtree as well as git-submodule (and possibly git-subrepo), and allow for passing options to the subtree/submodule command (e.g. if someone wanted a git subtree pull --squash to be the PR).

It's unclear that this should live in this precise repo rather than an adjacent one, but putting it here to track/ack the request.

[notEthan]

You can configure dependabot to update git submodules. .github/dependabot.yml:

version: 2
updates:
  - package-ecosystem: gitsubmodule
    directory: "/"
    schedule:
      interval: daily

or so. No subtrees though (dependabot/dependabot-core#3741), and I hadn't even heard of git-subrepo, looks like it is a third-party addon.

[Julian]

(Just seeing the comment)

That sounds perfect I think, and I'd forgotten about it -- changing the title to instead just say we should document exactly what you wrote pretty much.

PR welcome from anyone of course to do so.