Door SLAMS shut, I want to add resistance or SLOW it down

You need a "door closer"

A mechanical /hydraulic device that controls the rate of closure.....usually top mounted......like any commercial door

Here's an example .....you need to match the general weight of the door to the specs.......

http://www.acehardware.com/product/index.jsp?productId=1297983

"...measure once.....cut twice....throw that one away and cut a new one...."