Add the built-in factors to the API docs for zipline.io, so that I can link to the Returns factor from the release notes.
Add the built-in factors to the API docs for zipline.io, so that I can link to the Returns factor from the release notes.

bartosh/zipline,bartosh/zipline,florentchandelier/zipline,humdings/zipline,grundgruen/zipline,enigmampc/catalyst,magne-max/zipline-ja,Scapogo/zipline,Scapogo/zipline,grundgruen/zipline,quantopian/zipline,alphaBenj/zipline,alphaBenj/zipline,nborggren/zipline,umuzungu/zipline,enigmampc/catalyst,wilsonkichoi/zipline,nborggren/zipline,quantopian/zipline,humdings/zipline,umuzungu/zipline,magne-max/zipline-ja,florentchandelier/zipline,wilsonkichoi/zipline