Add to "How to Contribute" checking git artifact that PDFs knit (and anything else someone should know/do when contributing)

[lemireg]

^^ is there a benefit to checking git artifacts to see things knit (binary yes/no) (all that is checked in tests right?), or is the benefit of git artifacts only in seeing the content of the PDF.

This may just be an extra line in the "How to Contribute" page: https://github.com/FredHutch/VISCtemplates/blob/main/CONTRIBUTING.md

[slager]

Yes, the unit tests should fail if a report doesn't knit. The main intent of the artifacts is to be able to visually inspect the knit documents, and to be able to look at the intermediate/log/test files when debugging.