Deprecate build.sh script
Deprecate build.sh script

If `powershell` is installed, run `Start-PSBuild` to ensure DWIM. Else,
direct user to Linux / OS X build documentation.

Resolves #767.

bingbing8/PowerShell,kmosher/PowerShell,bmanikm/PowerShell,bingbing8/PowerShell,daxian-dbw/PowerShell,KarolKaczmarek/PowerShell,bingbing8/PowerShell,bmanikm/PowerShell,PaulHigin/PowerShell,PaulHigin/PowerShell,bmanikm/PowerShell,daxian-dbw/PowerShell,jsoref/PowerShell,KarolKaczmarek/PowerShell,KarolKaczmarek/PowerShell,PaulHigin/PowerShell,bmanikm/PowerShell,jsoref/PowerShell,jsoref/PowerShell,jsoref/PowerShell,daxian-dbw/PowerShell,kmosher/PowerShell,TravisEz13/PowerShell,bingbing8/PowerShell,KarolKaczmarek/PowerShell,JamesWTruher/PowerShell-1,KarolKaczmarek/PowerShell,JamesWTruher/PowerShell-1,bmanikm/PowerShell,kmosher/PowerShell,kmosher/PowerShell,TravisEz13/PowerShell,PaulHigin/PowerShell,daxian-dbw/PowerShell,jsoref/PowerShell,JamesWTruher/PowerShell-1,JamesWTruher/PowerShell-1,TravisEz13/PowerShell,TravisEz13/PowerShell,kmosher/PowerShell,bingbing8/PowerShell