Add CONTRIBUTING.md
Add CONTRIBUTING.md

Extracted from the current README.md

mdhender/hugo,jygastaud/hugo,Zenithar/hugo,anthonyfok/hugo,ffrizzo/hugo,sprig/hugo,gohugoio/hugo,tischda/hugo,coderzh/hugo,anthonyfok/hugo,MarkDBlackwell/hugo,fedelibre/hugo,derekperkins/hugo,fedelibre/hugo,coderzh/hugo,MarkDBlackwell/hugo,tischda/hugo,fedelibre/hugo,abourget/hugo,vbatoufflet/hugo,jygastaud/hugo,mitchchn/hugo,mayah/hugo,ifraixedes/hugo,mdhender/hugo,bogem/hugo,gohugoio/hugo,mayah/hugo,ffrizzo/hugo,soundrussian/hugo,mdhender/hugo,vbatoufflet/hugo,Zenithar/hugo,mdhender/hugo,digitalcraftsman/hugo,tubo028/hugo,ifraixedes/hugo,digitalcraftsman/hugo,sprig/hugo,melo/hugo,gohugoio/hugo,anthonyfok/hugo,derekperkins/hugo,abourget/hugo,mitchchn/hugo,tubo028/hugo,bogem/hugo,soundrussian/hugo,fedelibre/hugo,melo/hugo