Add dependabot

[oriontvv]

This PR adds dependabot config

[weiznich]

Thanks for opening this PR. Can you explain whether or not that will preserve currently specified minimal versions and how that will support version ranges (like those for the libsqlite3-sys crate)? Changing these minimal versions is not possible without issuing a major release.

[Eveeifyeve]

Even tho I am not the author but based on my uses and what the commit does it's doesn't but you can specify versions as I think. But really all it does it opens pr with the version updated you could just close the prs that you don't want.

[weiznich]

Given that we use version ranges in many places that would mean that we get a lot of PR's that need to be closed. I personally rather spend the time to do manual updates before any release instead of closing a never ending stream of PR.

cc @diesel-rs/reviewers for your opinions

[kw217]

I agree. I've used dependabot previously in a large organisation with many repositories. It was a helpful way of ensuring the work got done and nothing was forgotten, but I also had a dread of arriving in the morning to a pile of PRs. As long as Diesel has a process that ensures this work is done, I don't see the need for change.

[weiznich]

There seems to be a conses to not setup this integration.