Add initial contributing guidelines
Add initial contributing guidelines

These are modeled on the guidelines used by the
fabric8-analytics project:

* https://github.com/fabric8-analytics/fabric8-analytics-common/blob/master/CONTRIBUTING.md

mfranczy/prototype,ncoghlan/prototype,vinzenz/prototype,pcahyna/leapp-to-prototype,ncoghlan/prototype,pcahyna/leapp-to-prototype,GAZDOWN/prototype,vinzenz/prototype,vmindru/prototype,vmindru/prototype,GAZDOWN/prototype,GAZDOWN/prototype,pcahyna/leapp-to-prototype,GAZDOWN/leapp,vmindru/prototype,GAZDOWN/leapp,leapp-to/prototype,vinzenz/prototype,leapp-to/prototype,vmindru/prototype,mfranczy/prototype,leapp-to/prototype,ncoghlan/prototype,pcahyna/leapp-to-prototype,leapp-to/prototype,GAZDOWN/prototype,mfranczy/prototype,ncoghlan/prototype,mfranczy/prototype,vinzenz/prototype