Create dependabot.yml

[BNAndras]

@ErikSchierboom, is this something we should merge in? I took a quick look at Python's test runner since they're usually on top of things. They don't appear to have dependabot set up in that repo.

[ErikSchierboom]

@BNAndras It's up to you. Most tracks do merge this, as it is a relatively easy way to get PRs to update things.