README.md: Document x86-64 platform requirement
README.md: Document x86-64 platform requirement

See Homebrew/linuxbrew#339

princeofdarkness76/linuxbrew,outcoldman/linuxbrew,hyokosdeveloper/linuxbrew,caputomarcos/linuxbrew,davydden/linuxbrew,adamliter/linuxbrew,saketkc/linuxbrew,DoomHammer/linuxbrew,drbenmorgan/linuxbrew,caputomarcos/linuxbrew,arcivanov/linuxbrew,drbenmorgan/linuxbrew,jack-and-rozz/linuxbrew,amenk/linuxbrew,danabrand/linuxbrew,IsmailM/linuxbrew,dholm/linuxbrew,princeofdarkness76/linuxbrew,ebouaziz/linuxbrew,drbenmorgan/linuxbrew,Homebrew/linuxbrew,sjackman/linuxbrew,Homebrew/linuxbrew,Linuxbrew/linuxbrew,davydden/linuxbrew,tdsmith/linuxbrew,bukzor/linuxbrew,outcoldman/linuxbrew,afdnlw/linuxbrew,gcstang/linuxbrew,dholm/linuxbrew,danabrand/linuxbrew,jbaum98/linuxbrew,lvicentesanchez/linuxbrew,reelsense/linuxbrew,LeonB/linuxbrew,afdnlw/linuxbrew,saketkc/linuxbrew,IsmailM/linuxbrew,Gui13/linuxbrew,sjackman/linuxbrew,saketkc/linuxbrew,Angeldude/linuxbrew,afdnlw/linuxbrew,Homebrew/linuxbrew,caputomarcos/linuxbrew,Linuxbrew/linuxbrew,bukzor/linuxbrew,SuperNEMO-DBD/cadfaelbrew,Linuxbrew/linuxbrew,danabrand/linuxbrew,romejoe/linuxbrew,tomas/linuxbrew,Angeldude/linuxbrew,saketkc/linuxbrew,Angeldude/linuxbrew,danabrand/linuxbrew,tghs/linuxbrew,digiter/linuxbrew,tjhei/linuxbrew,lvicentesanchez/linuxbrew,jack-and-rozz/linuxbrew,LeonB/linuxbrew,afdnlw/linuxbrew,tghs/linuxbrew,caputomarcos/linuxbrew,jwillemsen/linuxbrew,gnubila-france/linuxbrew,outcoldman/linuxbrew,jwillemsen/linuxbrew,ebouaziz/linuxbrew,jack-and-rozz/linuxbrew,ebouaziz/linuxbrew,Gui13/linuxbrew,hyokosdeveloper/linuxbrew,digiter/linuxbrew,DoomHammer/linuxbrew,tdsmith/linuxbrew,gnubila-france/linuxbrew,lvicentesanchez/linuxbrew,gcstang/linuxbrew,tjhei/linuxbrew,tdsmith/linuxbrew,sjackman/linuxbrew,adamliter/linuxbrew,gcstang/linuxbrew,amenk/linuxbrew,reelsense/linuxbrew,princeofdarkness76/linuxbrew,lvicentesanchez/linuxbrew,davydden/linuxbrew,digiter/linuxbrew,tghs/linuxbrew,sjackman/linuxbrew,gnubila-france/linuxbrew,tdsmith/linuxbrew,tomas/linuxbrew,romejoe/linuxbrew,amenk/linuxbrew,bukzor/linuxbrew,Linuxbrew/linuxbrew,DoomHammer/linuxbrew,arcivanov/linuxbrew,Gui13/linuxbrew,tghs/linuxbrew,bukzor/linuxbrew,amenk/linuxbrew,tomas/linuxbrew,SuperNEMO-DBD/cadfaelbrew,brevilo/linuxbrew,tdsmith/linuxbrew,Homebrew/linuxbrew,brevilo/linuxbrew,DoomHammer/linuxbrew,jbaum98/linuxbrew,tjhei/linuxbrew,SuperNEMO-DBD/cadfaelbrew,romejoe/linuxbrew,reelsense/linuxbrew,adamliter/linuxbrew,reelsense/linuxbrew,IsmailM/linuxbrew,tghs/linuxbrew,LeonB/linuxbrew,digiter/linuxbrew,jbaum98/linuxbrew,brevilo/linuxbrew,hyokosdeveloper/linuxbrew,arcivanov/linuxbrew,davydden/linuxbrew,gnubila-france/linuxbrew,digiter/linuxbrew,IsmailM/linuxbrew,hyokosdeveloper/linuxbrew,ebouaziz/linuxbrew,Gui13/linuxbrew,dholm/linuxbrew,jwillemsen/linuxbrew,Gui13/linuxbrew,adamliter/linuxbrew,tomas/linuxbrew,jbaum98/linuxbrew,drbenmorgan/linuxbrew,Angeldude/linuxbrew,SuperNEMO-DBD/cadfaelbrew