updating contributing.md based on suggestions from updates to adapter… (

#4356)

* updating contributing.md based on suggestions from updates to adapter contributing files.

* removed section refering to non-postgres databases for core contributing.md

* making suggested changes to contributing.md based on kyle's initial lookover

* Update CONTRIBUTING.md

Co-authored-by: Kyle Wigley <kyle@fishtownanalytics.com>

Co-authored-by: Kyle Wigley <kyle@fishtownanalytics.com>