Update contributing guide
Update contributing guide

This change adds a section about working on open issues to encourage
contributors to work on those.

imattman/dotbot,bchretien/dotbot,bchretien/dotbot,pulgalipe/dotbot,pulgalipe/dotbot,imattman/dotbot,StijnRuts/dotbot,bchretien/dotbot,StijnRuts/dotbot,anishathalye/dotbot,anishathalye/dotbot,pulgalipe/dotbot,imattman/dotbot,StijnRuts/dotbot