Added unsupported background-image mixin to docs
Added unsupported background-image mixin to docs

chriseppstein/bourbon,thoughtbot/bourbon,craftsmen/bourbon,san650/bourbon,jamadam/bourbon-docs-jp,jamadam/bourbon-docs-jp,Jazz-Man/bourbon,greyhwndz/bourbon,Jazz-Man/bourbon,nagyistoce/sass-bourbon.io,greyhwndz/bourbon,smithdamen/bourbon,zyj1022/bourbon,cllns/bourbon,zyj1022/bourbon,Nikeshsuwal/bourbon,ismarkunc/bourbon,jamesmenera/bourbon-libsass,jasonwalkow/bourbon,setie/bourbon,setie/bourbon,jasonwalkow/bourbon,nickg33/bourbon,jamadam/bourbon-docs-jp,mindeavor/bourbon,jamesmenera/bourbon-libsass,jamadam/bourbon-docs-jp,craftsmen/bourbon,thoughtbot/bourbon,nickg33/bourbon,smithdamen/bourbon,chriseppstein/bourbon,mindeavor/bourbon