doc(floor): document `static_files` API
doc(floor): document `static_files` API

jedisct1/nickel.rs,nickel-org/nickel.rs,cburgdorf/nickel.rs,Ye-Yong-Chi/nickel.rs,Ryman/nickel.rs,tempbottle/nickel.rs,jolhoeft/nickel.rs,lzpfmh/nickel.rs,BenKnigge/nickel.rs,Manishearth/nickel.rs,derdela/nickel.rs,anvie/nickel.rs,nikomatsakis/nickel.rs,jedisct1/nickel.rs,SimonPersson/nickel.rs,ixjf/nickel.rs