Documentation: getting-started tutorial for adding new software version checks

[zcutlip]

Not sure if this is practical, but it'd be nice if there was a walkthrough or other guidance for newcomers who are interested in submitting a PR to add a new software version check (or other checks really). I poked around in the project a bit, and I could probably sort it out on my own. But guidance on:

• getting set up
• building the project
• any easy-to-miss details when adding a new check
• testing

...might lower the barrier to entry