An easy way to refresh a pull-request adding a contributor

**Is your feature request related to a problem? Please describe.**
When multiple contributors are being added, a PR could become stale (i.e not up to date with master/conflicts with the master branch). We should have a way to auto resolve this.

**Describe the solution you'd like**
`@all-contributors please refresh this PR`

**Describe alternatives you've considered**
Alternatively, we could watch for `merge` event son pull requests opened by the bot, and refresh the PR:

**Additional context**