Add basic contributing content
Add basic contributing content

Fixes #1611

A draft contributing document was merged a bit early, so I fleshed it
out to cover some of the basics.

maurizi/otm-core,recklessromeo/otm-core,RickMohr/otm-core,maurizi/otm-core,recklessromeo/otm-core,clever-crow-consulting/otm-core,maurizi/otm-core,recklessromeo/otm-core,clever-crow-consulting/otm-core,RickMohr/otm-core,maurizi/otm-core,recklessromeo/otm-core,RickMohr/otm-core,clever-crow-consulting/otm-core,RickMohr/otm-core,clever-crow-consulting/otm-core