add git branching tutorial to git workflow page
add git branching tutorial to git workflow page

k4rtik/docs,ZacharyRSmith/docs,rchavarria/docs,ZacharyRSmith/docs,NobbZ/docs,arashbi/docs,arashbi/docs,NobbZ/docs,yurrriq/docs,ekse/exercism-docs,canweriotnow/docs,lfex/exercism-docs,alexclarkofficial/docs,k4rtik/docs,exercism/DEPRECATED.help.exercism.io,spazm/docs,yurrriq/docs,lfex/exercism-docs,canweriotnow/docs,alexclarkofficial/docs,exercism/DEPRECATED.help.exercism.io,spazm/docs,rchavarria/docs,ekse/exercism-docs