There's a merged pull request on the void-packages repository

CONTRIBUTING.md: add instructions for testing PRs
https://github.com/void-linux/void-packages/pull/38447

Description:
I've had to find these instructions time and time again for requesting testing from people, so I figured why not make it part of our documentation. Maybe it will help get more users involved with testing.

Open to suggestions on wording, grammar, missing info, etc.