Add Contributor Covenant as our code of conduct
Add Contributor Covenant as our code of conduct

The purpose of adopting this code of conduct is to signal to potential contributors that this is a project where they won't be abused or attacked in any way. It also signals to everyone else that such behavior will not be tolerated.

This COC is copied from http://contributor-covenant.org/

I'll leave this PR open for a week, in the case that people have concerns with the wording.
iuriandreazza/activeadmin,mauriciopasquier/active_admin,mrjman/activeadmin,bolshakov/activeadmin,vraravam/activeadmin,iuriandreazza/activeadmin,mauriciopasquier/active_admin,Ibotta/activeadmin,gogovan/activeadmin,javierjulio/activeadmin,iuriandreazza/activeadmin,varyonic/activeadmin,quikly/active_admin,whatcould/active_admin,cmunozgar/activeadmin,bolshakov/activeadmin,vytenis-s/activeadmin,javierjulio/activeadmin,mateusg/active_admin,mauriciopasquier/active_admin,mateusg/active_admin,varyonic/activeadmin,activeadmin/activeadmin,iuriandreazza/activeadmin,bcavileer/activeadmin,siutin/activeadmin,cmunozgar/activeadmin,chdem/activeadmin,activeadmin/activeadmin,mateusg/active_admin,deivid-rodriguez/activeadmin,iuriandreazza/activeadmin,mrjman/activeadmin,vytenis-s/activeadmin,senid231/activeadmin,senid231/activeadmin,javierjulio/activeadmin,activeadmin/activeadmin,wspurgin/activeadmin,wspurgin/activeadmin,quikly/active_admin,waymondo/active_admin,whatcould/active_admin,bcavileer/activeadmin,deivid-rodriguez/activeadmin,halilim/activeadmin,maysam/activeadmin,waymondo/active_admin,senid231/activeadmin,bcavileer/activeadmin,deivid-rodriguez/activeadmin,siutin/activeadmin,maysam/activeadmin,cmunozgar/activeadmin,vraravam/activeadmin,iuriandreazza/activeadmin,gogovan/activeadmin,vraravam/activeadmin,halilim/activeadmin,maysam/activeadmin,varyonic/activeadmin,whatcould/active_admin,Ibotta/activeadmin,chdem/activeadmin,iuriandreazza/activeadmin,siutin/activeadmin,chdem/activeadmin,quikly/active_admin,bolshakov/activeadmin,mrjman/activeadmin,waymondo/active_admin,SoftSwiss/active_admin,gogovan/activeadmin,Ibotta/activeadmin,halilim/activeadmin,SoftSwiss/active_admin,vytenis-s/activeadmin,SoftSwiss/active_admin,wspurgin/activeadmin