STL: Document updating/adding tests procedure

[SuperWig]

Simply put, add Billy O'Neal (@BillyONeal)'s comment somewhere #567 (comment)

Probably by updating "Tests: Coming soon" to "Tests: WIP" with current info as well as a link to that comment?

[AlexGuteniev]

Is this still an issue?

[CaseyCarter]

At the very least, it seems we should remove the "high priority" label from this three-and-a-half-year-old bug.

[StephanTLavavej]

Casey Carter (@CaseyCarter) notes that we've certainly had lots of contributors adding tests in the last 3 years so we should just close this issue. I've added personal notes for the Contribution Guidelines that I'll be writing up to more thoroughly explain how to add tests properly, so we don't have to individually teach every new contributor.