Add david-dm.org badges in `maintaining.md`.
Add david-dm.org badges in `maintaining.md`.

Handles the first step of the Release process at a glance.

Collinslenjo/ava,sindresorhus/ava,andywer/ava-ts,Collinslenjo/ava,avajs/ava,andywer/ava-ts,avajs/ava,BarryThePenguin/ava