update CONTRIBUTING.md to include an Intro to Git and GitHub section
update CONTRIBUTING.md to include an Intro to Git and GitHub section

bthiago/Cachet,elektropay/Cachet,alobintechnologies/Cachet,wakermahmud/Cachet,Mebus/Cachet,h3zjp/Cachet,gm-ah/Cachet,ematthews/Cachet,brianjking/openshift-cachet,alobintechnologies/Cachet,anujaprasad/Hihat,robglas/Cachet,MatheusRV/Cachet-Sandstorm,n0mer/Cachet,ephillipe/Cachet,pellaeon/Cachet,Mebus/Cachet,CloudA/Cachet,Mebus/Cachet,brianjking/openshift-cachet,withings-sas/Cachet,MicroWorldwide/Cachet,pellaeon/Cachet,0x73/Cachet,NossaJureg/Cachet,wngravette/Cachet,NossaJureg/Cachet,chaseconey/Cachet,MatheusRV/Cachet-Sandstorm,SamuelMoraesF/Cachet,sapk/Cachet,NossaJureg/Cachet,bthiago/Cachet,displayn/Cachet,whealmedia/Cachet,leegeng/Cachet,ematthews/Cachet,billmn/Cachet,gm-ah/Cachet,ephillipe/Cachet,elektropay/Cachet,karaktaka/Cachet,elektropay/Cachet,eduardocruz/Cachet,MatheusRV/Cachet-Sandstorm,SamuelMoraesF/Cachet,coupej/Cachet,katzien/Cachet,eduardocruz/Cachet,g-forks/Cachet,cachethq/Cachet,clbn/Cachet,whealmedia/Cachet,h3zjp/Cachet,coreation/Cachet,katzien/Cachet,karaktaka/Cachet,bthiago/Cachet,eduardocruz/Cachet,coupej/Cachet,everpay/Cachet,MatheusRV/Cachet-Sandstorm,robglas/Cachet,ZengineChris/Cachet,brianjking/openshift-cachet,SamuelMoraesF/Cachet,ZengineChris/Cachet,murendie/Cachet,coupej/Cachet,MicroWorldwide/Cachet,murendie/Cachet,ZengineChris/Cachet,rogerapras/Cachet,karaktaka/Cachet,sapk/Cachet,coreation/Cachet,withings-sas/Cachet,everpay/Cachet,clbn/Cachet,anujaprasad/Hihat,sapk/Cachet,CloudA/Cachet,leegeng/Cachet,anujaprasad/Hihat,pellaeon/Cachet,cachethq/Cachet,rogerapras/Cachet,displayn/Cachet,0x73/Cachet,murendie/Cachet,g-forks/Cachet,Surventrix/Cachet,MatheusRV/Cachet-Sandstorm,wakermahmud/Cachet,n0mer/Cachet,n0mer/Cachet,Surventrix/Cachet,ephillipe/Cachet,rogerapras/Cachet,chaseconey/Cachet,billmn/Cachet,withings-sas/Cachet,billmn/Cachet,wakermahmud/Cachet,ematthews/Cachet,g-forks/Cachet,nxtreaming/Cachet,nxtreaming/Cachet,chaseconey/Cachet,h3zjp/Cachet,wngravette/Cachet