**Description**:
Add a new file `dependabot.yml` to keep the actions used in
`.github/workflows` up to date. This will automatically open PRs when a
new version of an action becomes available.
**Related Issue(s)**:
Implements #324
**NOTE TO REVIEWER/MAINTAINER**:
You may need to enable dependabot functionality in the settings on this
repo. I can help dig into that if you need help. Thanks!
Signed-off-by: Andrew Brandt <brandt.andrew89@gmail.com>
