brew.sh: complain when running Homebrew as root.
brew.sh: complain when running Homebrew as root.

As discussed: announce that we are deprecating running Homebrew as the
root user as it's insecure and requires disabling the sandbox.

Fixes #796.

rwhogg/brew,bfontaine/brew,JCount/brew,hanxue/linuxbrew,konqui/brew,toonetown/homebrew,claui/brew,ilovezfs/brew,zmwangx/brew,sjackman/homebrew,pseudocody/brew,alyssais/brew,nandub/brew,JCount/brew,konqui/brew,JCount/brew,bfontaine/brew,aw1621107/brew,reitermarkus/brew,vitorgalvao/brew,palxex/brew,rwhogg/brew,Homebrew/brew,muellermartin/dist,reitermarkus/brew,jmsundar/brew,amar-laksh/brew_sudo,gregory-nisbet/brew,zmwangx/brew,Homebrew/brew,maxim-belkin/brew,sjackman/homebrew,claui/brew,muellermartin/dist,nandub/brew,sjackman/homebrew,tonyg/homebrew,EricFromCanada/brew,nandub/brew,mgrimes/brew,sjackman/homebrew,alyssais/brew,claui/brew,amar-laksh/brew_sudo,hanxue/linuxbrew,reitermarkus/brew,aw1621107/brew,rwhogg/brew,pseudocody/brew,ilovezfs/brew,palxex/brew,maxim-belkin/brew,Homebrew/brew,amar-laksh/brew_sudo,Linuxbrew/brew,alyssais/brew,bfontaine/brew,tonyg/homebrew,reelsense/brew,konqui/brew,gregory-nisbet/brew,MikeMcQuaid/brew,reelsense/brew,mgrimes/brew,ilovezfs/brew,mgrimes/brew,pseudocody/brew,Linuxbrew/brew,nandub/brew,reelsense/brew,JCount/brew,EricFromCanada/brew,MikeMcQuaid/brew,toonetown/homebrew,EricFromCanada/brew,mahori/brew,DomT4/brew,claui/brew,DomT4/brew,gregory-nisbet/brew,konqui/brew,hanxue/linuxbrew,mahori/brew,DomT4/brew,tonyg/homebrew,Linuxbrew/brew,Linuxbrew/brew,mahori/brew,MikeMcQuaid/brew,vitorgalvao/brew,zmwangx/brew,vitorgalvao/brew,jmsundar/brew,vitorgalvao/brew,EricFromCanada/brew,mahori/brew,reitermarkus/brew,jmsundar/brew,aw1621107/brew,palxex/brew,DomT4/brew,MikeMcQuaid/brew,muellermartin/dist,Homebrew/brew,toonetown/homebrew,maxim-belkin/brew