Add .mergify.yml
Add .mergify.yml

Config for https://mergify.io/ to automatically merge PRs once all tests
pass and it has been reviewed.

In order to avoid merging before the author has given the okay, this
requires a new status check `mergify/allow` which has to be set
manually.

crate/crate,EvilMcJerkface/crate,crate/crate,crate/crate,EvilMcJerkface/crate,EvilMcJerkface/crate