refactor: improve contributing guidelines lists

CONTRIBUTING.md

@@ -38,15 +38,11 @@ We use GitHub issues to track public bugs. Report a bug by [opening a new issue]
 **Great Bug Reports** tend to have:
 
 -   A quick summary and/or background.
-
 -   Steps to reproduce:
     -   Be specific!
     -   Give sample code if you can. [My stackoverflow question](http://stackoverflow.com/q/12488905/180626) includes sample code that _anyone_ with a base R setup can run to reproduce what I was seeing.
-
 -   What you expected would happen.
-
 -   What actually happens.
-
 -   Notes (possibly including why you think this might be happening, or stuff you tried that didn't work).
 
 People _love_ thorough bug reports. I'm not even kidding.