Document topic branches
Document topic branches

Since we made the graph branch, and since I just moved the branch pointer unilaterally, I figured I ought to legitimize such things. This commit updates the contributing document to describe how I think topic branches like this ought to be handled.
adamnovak/schemas,diekhans/ga4gh-schemas,saupchurch/schemas,diekhans/ga4gh-schemas,benedictpaten/schemas,hjellinek/schemas,nlwashington/schemas,sarahhunt/schemas,dnanexus-rnd/schemas,ga4gh/schemas,buske/schemas,delagoya/schemas,buske/schemas,adamnovak/schemas,pcingola/schemas,dcolligan/schemas,laserson/schemas,hjellinek/jeltje-schemas-sphinx,heuermh/ga4gh-schemas,dnanexus-rnd/schemas,benedictpaten/schemas,macieksmuga/schemas,dglazer/schemas,lh3/schemas,Jeltje/ga4gh-schemas,hjellinek/schemas,nlwashington/schemas,kellrott/schemas,lh3/schemas,laserson/schemas,ga4gh/ga4gh-schemas,kellrott/schemas,delagoya/schemas,sarahhunt/schemas,Jeltje/ga4gh-schemas