Update documentation in contributing readme (#476)
Update documentation in contributing readme (#476)

* Update documentation in contributing readme

Include info about checking code coverage and more details around
writing visual tests.

* Doc tweaks

Noticed a misspelling, so I fixed that, removed an unnecessary comma, fixed a couple capitalization things, and tweaked some wording.

blackbaud/skyux2,blackbaud/skyux2,blackbaud/skyux2,blackbaud-joshgerdes/skyux2,blackbaud-joshgerdes/skyux2,blackbaud-joshgerdes/skyux2,blackbaud-joshgerdes/skyux2,blackbaud/skyux2